Bodywork is something that cannot be rushed. While others are suggesting chemicals that may be suited for this application, it is not necessarily going to be a solution to your problem. The most important part of bodywork/painting is preparation and cleanliness. I cannot stress this enough.
When applying bondo, fibreglass, spot putty, primer, paint, clearcoats, etc, temperature is key. So is the mixture of the product(s).
A paint booth must be a minimum of 55 degrees for any automotive paint
with a catalyst to dry. All automotive paint companies base the time
it takes to flash, cure, recoat, etc. on 70 degrees and 50% RH
(relative humidity). There is a standard basic rule in the automotive
industry that for every 15 degrees above 70 degrees, the coating will
cure about twice as fast. This rule works the other way around as
well; for every 15 degrees below 70 degrees, the coating will take
about twice as long to cure. Source.
While flex agents could certainly aid in this application, it looks like the paint did not properly adhere or moisture is being trapped.
Assuming you have the correct products, there is still much work to do.
Start with removing those flakes. I would suggest sanding with coarse grit, (40-80), in this case because you don't know the extent of the damage yet.
If you have access to air tools, great, if not, use a sanding block.
Feel free to to sand it with something finer if you wish. Before adding your filler, you must clean the surface.
You can do this with a product called Final Wipe, or equivalent.
Removes wax, grease, oil, silicone, tar, road film and other solvent based contaminants. Use on bare metal, existing finishes and primer surfaces as a final wipe before painting.
Finally you can lightly go over the surface with a Tack Cloth. It will catch any particles that you may have missed.
Rinse and repeat throughout your remaining stages. If you take all of this into consideration, you will prepare yourself for a successful job. It doesn't take long to spray a vehicle, or part, etc, but the time to get there is certainly worth putting in.