Why my c++ vehicle don't move?

hi all

I have a problem with my c++ vehicle.

I follow this tutorial:

for my project (verion 4.18) i must set the physicsbox for the vehicle itself and the physicspheres for the weels a littlebit different.

All work correct, the player get into the vehicle, the weels rotate correct but the vehicle stay still and don’t falling to the ground.

have anyone a idea why my vehicle stay still?