I've got the MK6 and I have replaced the red PTFE after too much use on the machine with new red PTFE and it seems that the red tubing, where the aluminum heater block is, is causing the red tubing to enlarge the inner diameter.
After the heater is at 220*C, maybe 2 minutes, I try to pull the filament out and push it back in a few times, which it should allow me to. But I can clearly see the filament has expanded. I have tried over and over to replace it with the same material to no avail.
Has anyone had this problem? I can take a photo if my explanation is difficult to read.