Printing trouble: Z axis squishing?  

Printing trouble: Z axis squishing?

So I finally  got around  to setting  up my  new mini and initially had some  issues  printing. I thought it was the  common extrusion problem  so I reseated the heat  break and replaced the PTFE tubing but the issue persists.   After printing a number of things with the exact same settings and filament off of the USB drive  provided, it seems like  at  random points t he  Z axis is not  moving and the hot end just digs in to the print as it spews out more filament.  I've tried updating to the newer 4.5 rc  firmware but I'm  getting the same results.  I've checked all  the screws to make sure nothing is loose as well as the belts.  I've also run this filament through my mk3 and it prints perfectly.  I'm a bit baffled since the whistle came out flawless and the nut has a few layer inconsistencies but the sheep and benchy got squashed. If anyone has any ideas let me know!