I have found that over time, material builds up on the inside of my nozzle causing a kind of clogging I cannot resolve. It can't even be called a clog really, the passage is actually clear. The problem appears to be that the material cannot heat up properly inside the nozzle because the walls are lined with goo. This means that while the printer can extrude, it requires far too much force to do so (extruder skips etc).
Keep in mind this particular E3D V6 nozzle has only ever printed PLA, though the goo is most likely comprised of dust and other foreign particles.
So far I have found no way to clean this out. Things that have failed:
So basically this slowly degrades the performance of my nozzles over time (with the usual number of "normal" clogs along the way) until the nozzle becomes useless. At that point I have to throw it away and start the cycle over with a new nozzle. If anyone knows what can be done about this, I'm all ears.
Here are some close-ups of a bad nozzle and a brand new one for comparison. Although you can't see it in these pictures the hole of the bad nozzle is actually clear, I can see through it with the naked eye and fit 0.4mm needles through.