The problem is that you bought tread that was cut for different wheels then you tried to apply it to.
The
6" IFI Traction wheels have a 5.6638" OD, while the
6" AndyMark FIRST Wheels have a 6" OD. Thus, the circumference of the AndyMark wheel is about 1" longer then the IFI wheel.
I can think of a bunch of different solutions:
Cut/grind the rubber from the AndyMark FIRST wheel to 5.6638" OD
Buy the 6" IFI Traction wheel and use the tread you already purchased
Buy the tread material directly from McMaster Carr and cut to size.
Buy either the AndyMark traction wheels or performance wheels