Some things that I can think of off hand that cause bubbles:
single layers
layup that is heavy on the egdes
no way out for the air
shelves with a dip
bubble squeeze at the wrong temperature – too low or high
If you give us some more information on configuration of kiln and of glass layup, schedules and all those other variables, we might be able to suggest some things.
Stephen Richard
blogs at: http://www.verrier-glass.blogspot.com/ and http://www.glasstips.blogspot.com/