Humbucker selector switch problem

I am stumped! I just acquired a 25 year old Guild X-150D. It is great but I now have a problem. It has 2 humbuckers, 2 volume and 2 tone pots. When I select either pickup it works fine. When I select BOTH pickups (switch in centre position) only the bridge pickup plays. I have pulled out the switch, examined it and tested it with an ohmmeter. It tests fine on the meter. Both pickups connected. Any ideas?????