Hmmm. Try this:

1. Partially deflate your tires. For this you'll want them almost flat.
2. Push the valve stem straight downward. 1/4" will be plenty.
3. Now reinflate your tires.

The inner tube immediately adjacent to your valve stem is a little thicker than the rest of the tube. If that thick section gets caught under the tire beads, the beads won't seat against the rim like they do over the rest of the tire.