Seems like you should be able to temporarily tape down a small piece of plastic of the appropriate size to help prevent the roller from dropping down into the gap, if that's what's happening. Just make sure it doesn't interfere with the movement of the switch point when in the divergent position.
If it works, then glue it in place.
If you have a spare center rail, you might also be able to cut it to just slightly longer than the original piece and replace it.
