Rigid Dynamics: drive wheels spin but chassis won't move up 5° ramp (works fine on flat ground)

Dimple

https://image2url.com/r2/default/images/1785556229693-2fb91107-1188-4d7e-a1de-7fe33f9c0f5f.png

I'm running a Rigid Dynamics transient simulation of a two-wheel differential-drive robot climbing a 5° ramp, to validate a hand-calculated drive torque requirement. The same joint/contact setup works correctly on flat ground, but fails specifically on the incline.