My E3Dv6 has started having issues on retracts lately, after working flawlessly for months, and I cannot for the life of me figure out why. Whenever there are lots of retracts, the filament simply stops coming out of the nozzle. When I heat up the hot end after this has happened, the filament pushes through without any issues, so I guess it's not technically a jam, but it has the same effect. I've tried the following things: Retraction length 1, 2, 3 and 4mm (bowden setup, I usually have 4mm retraction length. Lowering both print speed and retraction speed to very low settings. Changing the whole hot end to another v6. Changing the extruder twice (I used this as an excuse to buy a Titan which I have mounted now, same problem). Tested several different rolls of PLA and one roll of ABS. Tried different temperatures for PLA (190 and 210, I usually print at 210). I also changed the hot end mount since I normally use toolsons mount (http://www.thingiverse.com/thing:953811) that doesn't use the stock fan shroud, so I switched to one that does. I might have tried more things that I can't think of right now, but nothing seems to have any real effect.