if all you did was replace the tips themselves you may want to go back and double check the insulators for wear. they do wear on one side after a while this can prohibit the movement of the armature and will not allow full travel of it. ultimately causing the tips to not make good contact or not contact at all.
if you can access the tips have someone get on the truck, (make sure the drive wheel is off the floor)
actuate the system without letting it move(in creep speed), wiggle the tips and see if the truck tries to move.
this will verify this problem.
if the contact area is enclosed and non-accessible then you just have to wing it i guess and take them apart.