From your question I think you mean that there are air bubbles under the decals (between the decal and the covering)? If this is the case try making a pin hole in the decal and with a warm iron try pressing the decal down working towards the pin hole to release the trapped air.

If the bubble is under the covering try the fine pin hole through both decal and film. Heat very carefully with the heat gun and with a glove (to protect your hand) work the bubble towards the pin hole.

With either situation work slowly and methodolically and use the minimum heat. Too much heat and the decal will shrink and lift.

Good Luck,

Colin