The main Growing tomatoes forum has a good FAQ entry on blossom drop; you might start there. My personal thought is that you can try flicking or vibrating blossoms, or shaking the plant a bit every couple of days; the humidity may be causing the pollen to clump together so it doesn't fall down to pollinate the flowers.

Blossom drop is one of those problems with many causes, so it takes a bit of detective work to figure it out.

--Alison