M300 Z-ESD Extruder Slipping HELP


We have a M300 printer and have recently had an issue with printing Z-ESD V2, we did not have a problem with this until recently.

The problem is the extruder gear keeps slipping as if the filament is clogged, this happens very early on in the the process, sometimes even during loading the material.
I have replaced the motor, motor gear, bearing and cable. This did not solve the issue. I have also replaced the hot end with both V1 and V2 but yet still no luck, and tried different nozzles.

I have also double checked the bed levelling.

It appears to be getting stuck just after the extrusion hole and just before the hot end entrance, after inspecting the filament that was stuck it appears the diameter has increased ever so slightly.
I cannot figure this out, any suggestions would be great.