I think you need to keep the resin under pressure for a longer period of time. Best I can remember it is recommended to keep the pressure on for at least 2 or 3 hours, but from the IAP some did it longer. The resin may look and feel solid, but the fully cure time is around 72 hours. That air bubble is kept small as long as there is pressure on it. If the resin hasn't set hard enough and you release the pressure the air bubble can or will expand.
Don't take this to the bank, it has been a few years since I quit playing with resins. All of this info is off my own memory bank.
Marvin