Changed battery and USB but not turning on

Not too long ago my HTC M8 simply stopped charging or turning on so I've used the ifixit guides to swap out the battery and USB module (using Nitrile gloves, and an anti-static tray).

Since replacing these, the phone now indicates it is plugged in by showing a flashing red/orange light at the top and the charging picture on the screen, but only if the plug is held toward the back.

It's been kept overnight in a position that leaves it showing as charging, and again most of the morning since getting into work (it's now 11am, been charging over 2 hours since getting in) but it won't switch on, and if I recall correctly it is charging when the red light is solid, not flashing.

I've only just tried the method of holding all 3 buttons for 2 minutes to reset the charging circuit and put it back on charge to see if it finishes charging before I finish work today, but as of yet it still doesn't switch on.

Can anyone suggest what else I can check? Should I try replacing the motherboard as well? I've got this, and an HTC-built Nexus 9 both died within weeks of each other and i'm desperate to get a working device again.