Well placed toothpicks should do the trick.
IIRC, you only need 2 (as there are only 2 latches on the chassis). Having the loco upside down with the front facing away from you, place one toothpick on the right side in the front and on the left side at the rear. But 4 toothpicks won't hurt either. I suspect that you are just placing the front toothpick in a slightly wrong location.