Jack,
Not letting the solvent evaporate in the underlying layers will cause this problem. The top layer skins over and traps the solvents. And eventually the trapped solvents weaken the under layers, adhesion fails and viola you have big nasty blisters. If the ambient temperature is a little higher than acceptable the clear will skin over that much quicker.
As may have already figured out stripping the finish and starting over is your only recourse.
I hope I was able to help you in some way.
Good luck with your future refinish efforts.
sscuda.
