I'm using the Knights on Wheels sample in the A8 full featured demo and when I change the car to another low poly model it works just fine, however, if I replace that low poly with a larger detailed one I see the back tires spin and the front tires turn but it never moves from the original location. Any ideas?

maybe the origin of your model is too low and thus its touching the ground, open it in MED and try moving it up a bit...

I moved the origin higher, and tried changing the location of the tires and the Z value of the tires and model - so far nothing has worked.