If the transmission then a problem. You could check that by taking off a wheel and see if the drive axle is turning when it’s supposed to.
It’s possible for the issue to be out at the wheels. The drive axle turns when engaged. There is a keyed slot out at each end of the axle. There is a small hole and spring under each key. The springs push the key up to catch the drive gear. The drive gear drives the teeth on the inside of the wheel. Those components are held in position by two clip rings.
You can take the wheel assembly off and see what is going on in there.
The plastic wheel teeth could be the problem.
The assembly may be out of line due to a clip ring falling off.
The assembly may be out of line due to the wheel position catch being away from the catches on the wheel height bracket.
The springs may be rusted and the key sticking in low position not catching the drive gear.
Taking the wheel off and looking around in there will probably expose what is wrong.