|MTL SD40-2 Truck alignment help

I've got two of these and I can't seem to get the rear trucks to stay engaged with the worm gear on either one of them. Any help would be appreciated.

Try to take off the trucks and reinstall it again. While they are out, roll them to see if they move freely. Never had a truck that didn't engaged to the worm gear. Come to think of it, I did have one, but that was a brass loco.