Custom Vehicle sinking in the ground & not moving

I have a custom vehicle (a snow mobile), and no matter what I try the wheels do not collide with the ground, and it does not move.

The setup is that I have 6 wheels in the back (3 left, 3 right), and 2 wheels in the front (the skis). You can see that for the bones, Z is up, X is forward.

Here’s the PHAT:

And the simulation:

In game, the vehicle sinks:

If I enable rendering with pxvis collision you can see here that the wheels are properly aligned:

I’ve also tried disabling collision for the skis, nothing changes.

Any ideas on what I could try…?

If anyone should find the same issue: I simply had to limit the wheels to 4, and have the other 4 wheels copy bone animation.