Advice on mk7 gear set screw problems (no replies)

Hi guys ths problem s doing my head in

I'm using i3 extruders that use the mk7 gear pully to grip the filament

The problem I have is my stepper motor skips , filament stirs and skips the lot

I rip the extruder out and look at the gear and the hobbled gear has worked only a bit loose and moves around 1mm on the 5mm bolt

I have problems getting the m3 set screw to old good.

When I tighten down n the set screw I either round off the screw or the hex key.

If also tried tread lock

Only thing I can imagine is te m5 hex head bolt s soft metal and thats the problem

Any advise??