It could be a bad solder joint, even though it may look ok, or if it's a cheap switch it could be burnt out from soldering.
This happened to me recently when I tried a few different pickups in my Ibanez RG. I replaced it with good quality switch (one I got free from BKP but didn't need to use) and have had no problems since then.
Edit: If you have a multimeter you could try checking continuity between the middle pickup tag on the switch and the output tag. If you don't have a multimeter, get one! 8)