Summary

The 'stock' ones were too thick for the t-nuts to clamp the extrusions well, even when cranked so tight that the plastic started to crack. (You could move most of them by hand!)
They were also PLA, and they melted with the heat of the motors. (See the pic with the black part)

I simply thinned the area where the t-nuts mount to give more room for them to clamp, plus I printed my new parts in ABS so they would be more resilient to heat.

When assembling, I also used longer screws to mount the motors, with washers on the heads and cork gaskets on the stepper motors. (This provides insulation from a warm motor, and reduces noise too)

*** Some may say I was running my motors at too high of a current, and that may be, but they were skipping at any lower setting. The pulleys that were provided were 36T so they required more torque. When I switched to my new corners that I printed, I also changed to 20T pulleys and lowered the current until the motors did not slip with the 20T pulleys. This also gave me finer resolution. (Note that I was using 29T 'GeeTech' pulleys after installing my new corners, but found my prints were skewed quite a bit and found the 29T pulley teeth did not mesh well with GT2 belt. So I ordered some 20T pulleys and my prints are AWESOME now!)