Thanks for all your replys. I've been doing everything you've syggested. I think I know the problem - it could be uneven pressure. I adjusted the press for 3/16 board, but when I ran out of this board I purchased 5/16 and did not re-adjust the pressure. Looking at the areas where the bubbles are - they are all at the adge of the prints, so this could be dues to uneven pressure.