Hot End Jams
My heavily used Prusa Mk3 developed a tendency to after running OK on a print for a while start clicking the extruder with if left eventual cessation of filament feed always well into a print time span.
The end of the filament when ejected was larger for maybe 2-3mm back.
This would repeat, the new print would run OK for some time (over an hour even!), then clicking started and eventually the filament feed stopped completely.
It could be got round by catching it before it stopped totally, print pause, unload filament, re-trim and reload/end pause.
However, it might occur again on that print if there were enough time left for the fault to recur as it only happened after at least 20 mins good run time.
I had changed the hot end two months or more before when I needed to change the nozzle and found it completely seized in, so bought a complete pre-assembled unit, which had worked well for many prints until this fault. (The old one, now fixed became a spare)
On dismantling it all this time I realised that the rectangular block with nozzle/thermistor was a few turns slack. a few mm down from the heatsink body.
Could not see how the two could get like this after fitting and in use, so assume it was like it when supplied and I missed seeing it. (Remember, it was an assembly supposedly ready to fit and was not loose when a nozzle fitted to give any clue of being wrong)
Removed hot end, Tightened up, and replaced.
Then of course the Z height was completely screwed up so had to go back right through bed levelling, Pinda, nozzle first layer height setup.
However...........I'm now several perfect prints later............... and its running a dream.