I am using the MK8 gear but I'm using Roberts feeder.
If it just appears not to get a grip on the filament (as in: you barely see teeth marks in the filament) it might be that you have to up the tension (there's a small screw on top of the feeder for that).
But like gr5 said: Make sure its not grinding or that there is something else wrong (ie if you hear a "ploing" sound once in a while it's the feeder skipping backwards because of too much pressure)
Is it the filament that is slipping or is the feeder turning backwards?
Check that there isn't ground up PLA in the teeth of the mk8. More information here:
You might want to stick with the original feeder sleeve.
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