Ok, so whats wrong?

[Neale]

Things that occur to me, in no particular order and with apologies if you've already checked...

Is the hot end really at the temp it says? I'm not sure what PID controller you are using but there are various common thermistors in use and they are all calibrated differently. In the end I calibrated mine by taking out the filament and shoving the business end of a thermocouple down the filament hole so I was reading temp actually inside the nozzle.

Looks like a direct drive from stepper to toothed drive roller (or whatever it uses) which does need a bit more urge on the stepper than a geared drive but if it's slipping, then there is enough drive but not enough grip (maybe). Can you release the pressure/grip on the filament and just shove it by hand? I'm more used to 3mm filament but you can get it to ooze through the hot end with firm hand pressure and I would expect your 1.75mm to do the same.

I'm guessing that you have calibrated the extruder by telling Mach3 to push out a known length of filament, and checking that it has done so? Was that with the nozzle removed? Current versions of Slic3r expect this to be the length of filament going in; older (much older) versions used the length of extruded filament coming out which wasn't that easy to measure... I've no idea what values you would expect to see in Mach3 but acceleration on the extruder motor is a pretty meaningless quantity, I suspect, as the motor (especially with direct drive) hardly turns at any real speed.

Can you measure the actual nozzle diameter (by sticking a bit of fine wire or drill up it)? It isn't blocked, maybe?

Is the filament easy to push through with no roller pressure or nozzle? In other words, it's not oversize for the feed tube, etc?