Plarail Advance series 165 reverse function issues

Is it simply that the switch is fouling on the train body and not allowing the switch to make contact when you try reverse?  If so it may be you need to file the slot to make it a bit wider and allow the switch to move further...

I've not come across this problem before, but if it works fine without the body on its the only thing that springs to mind.  Could you take some pictures of the switch unit with the cover on in all positions?  You know forwards/off/reverse...