If you've changed the inner tube, is it bunched up in one place? When I'm in a hurry, I don't put the inner tube back in the tire correctly, and it ends up being a bit of a mess with the inner tube bunched up just at the valve stem. What the Park Tool guy told us to do is deflate the tire, push the valve into the tire, then pull it back out and reinflate. It always works for me.

Koffee