You did not mention the primer that you are using, the surface that you are painting over (epoxy, or polyester resin) and how you cleaned the airplane.

If you are using the PPG K-36 primer, clean the surface first with DX 330 wax and grease remover. Let it dry, you can accelerate the drying by using an air gun. You might try spraying several very thin coats of the K-36 over it after cleaning very well. Clean it again between coats.

Is it possible that the glass was not fully cured before priming?