Are you being hexed by failing print jobs? Here is my list of the most common reasons for filament jamming.
If you get "air prints" or "wispy prints", it's because your extruder has stopped extruding plastic -- either because your nozzle has jammed, or there is a problem preventing the extruder from feeding the filament.
If you have a horizontal gap in your printed object, where the object can easily pull apart, you had a temporary jam that recovered.
If you regularly run high-value print jobs, consider installing the Tunell Filament Monitor. By automatically pausing the printer as a filament feeding problem is detected, the monitor gives you a much better chance to rescue your prints.
While I originally prepared the following list for Makerbot operators, much of this applies to other FDM/FFF style printers.
Backpressure at the nozzle when you over extrude
If you set the wrong filament diameter for the slicer, you end up laying down a thicker layer of plastic than expected. Over time, that excess will accumulate, the print-in-progress will be much taller than expected, and the gap between the extruder and the layer below will until there's no room for the extruded filament to go.
Sometimes, if you leave the printer alone, as the extruder moves up a layer or two, it might start extruding again if the extruder was able to grab filament.
BTW, if your print is failing at the very beginning, your nozzle may be too close to the printbed. The same mechanics (no room to extrude the plastic) is in play there.
On some printers, when there is too much back-pressure at the nozzle, the filament drive gear may sometimes skip backwards and relieve the pressure. When this happens, there may be momentary stop to the flow of filament before resuming, resulting in a repeated pattern of gaps within the same layer.
Solution: increase the filament diameter (i.e., reduce filament feed volume) when slicing to extrude less plastic.
Wild swings in temperature
Your extruder temperature is being controlled through a PID algorithm that attempts to steadily heat the nozzle to maintain a consistent temperature. The problem is that the amount of filament going through the nozzle is not always steady -- your particular print may alternate between high-duty-cycle and low-duty-cycle areas. This seems to particularly affects PLA prints, and the Replicator 2X seems to have it the worst.
My solution: turn on a box fan and blow it at the printhead -- the resulting thermal flow into the flowing air will dominate the extruder's heat transfer, so that changes in plastic flow rate will have less effect on the hotend temperature. Installing a "nozzle cooler" upgrade will accomplish the same thing.
For advanced Makerbot operators, if you have started experimenting with deliberately changing temperatures mid-print, you may be hit by a bug in the PID controller code which resets the PID and temporarily leads to over/undershooting.
Proper solution: better PID tuning. Even better (this next part needs changes in the design of the printer controller) feed-forward correction for heating the hotend.
Excessive retractions in the same spot
If you have a lot of retractions in between small features, the same spot on the filament can get chewed up enough to the point where the extruder can't effectively push the filament anymore, and then it just keeps grinding away at the same spot after the filament is no longer moving.
Solution: you might try setting/adjusting the restart extra distance. Unfortunately, that could lead to blobbing at the restart point.
Filament infeed problems
It's pretty obvious that the filament will stop if it's tangled or knotted while on the spool, or if the filament came off the spool and got wrapped around the spool holder.
Solution: make sure that your spool is not knotted. I still have to take a video on how to do this. I have to get someone to work with me on that -- I just feel silly taking videos of myself with my own camera.
Filament feedpath problem
The feed tube friction may be too high from wear (not likely with with a new machine, but I know at least one person had worn his tube enough to need replacing)
Solution: replace feed tube, or go to a tube-less arrangement.
Or, you mounted filament incorrectly.
Solution: Mount filament to avoid sharp turns or other sources of resistance.
Extruder drive problem
Have you checked to see that your extruder is doing a good job pushing filament?
On the Makerbot's, the original Delrin plunger design has a reputation for being very sensitive to the tension adjustment being set just right for the incoming filament, and for not being able to tolerate filament diameter changes well. Worse, the Delrin plunger would wear down over time, requiring readjusting the tension.
Solution: Upgrade your extruder with a spring-loaded idler-arm with a metal bearing to hold the filament against the drive gear. Note: Makerbot recently announced free upgrades for Replicator 2 owners. Or, you can go with an aluminum upgrade like the ones sold here.
Accumulated dust/dirt on the filament, or even actual debris inside the filament, can partially or fully block the narrower output end of your nozzle.
Solution: clear nozzle, ensure filament is clean. Use a filament "wiper" to remove dust that have accumulated on your filament. I just wedge my filament inside a folded over paper towel in a clothespin.
Filament quality problems
If your extruder is sensitive to the diameter of the filament, it may be have problems if your roll of filament varies widely in diameter.
In some cases, the diameter variation is gradual -- you might have great prints for a long time, and then as the filament gradually gets narrower, you have a extruder drive problem. Or, as the filament gets wider, you start to have an over-extrustion problem.
Solution: For slow-changing diameter, you might be able to get by with adjusting the extruder tension or the slicer setting to take the new filameter diameter into account. If the changes are fast (happens every few changes), it might be more difficult to compensate for this.
l have read some reports of a big bulge in the filament where two ends of filament were apparently spliced together. Worse, there have been a report of a knot where two filament were simply tied together, and another where the filament ends were taped together). If this was the cause, you should be able to spot the actual obstruction. This is actually a sign that the roll of filament was not originally manufactured for 3D printing, and is more likely industrial plastic welding rod that was simply rebranded for 3D printing.
Solution: remove obstruction if it's an isolated incident, and carry on. Better, switch to higher quality filament (like the filament that we sell) that is made specifically for 3D printing.
I actually have a hypothesis which I think may be another likely reason: the room is getting too warm.
If your shorter print jobs always seem to go well, but longer ones seem to consistently fail after running for X hours, there must be something that's changing in that time period to cause the problem. Given that that pretty much everything in the printer should have reached its steady-state temperature, it's hard to imagine problem surfacing more than 30 to 60 minutes into the print. But, if you are printing in an enclosed area, the printer can act as a space heater and, eventually, the air temperature is much higher than when you started.
The room temperature could affect the printer either by directly affecting the extruder or possibly by affecting the accuracy of temperature measurement. (There was recently some information to suggest that Makerbot's thermistor measurement implementation may be slightly broken, for example.)
This is an educated guess, however, and not a well tested theory. If Makerbot or anyone else would like to let me borrow their 2X for a while, I'd be happy to run some tests.