What product did you use on the repairs?
Are the cracks that are showing up now the same ones you repaired?
How long did the bumper sit between the repairs and priming, and between priming and painting?
The bubbling in the paint could be 1 out of 3 things
Poor adhesion of repair material to bumper substrate which could be caused by dust dirt or solvents used for the prep.
Air pocket in repair material .
Solvent evaporation from repair material, not throughly mixed or sanded and primed before repair product was fully cured.
If you are up to it.....I know it's tough...take a sharp pointed knife and start digging into the bubbles a little bit at a time.
You should be able to get to the root of the problem. Keep digging until you get to solid material. This way you will know how far back you need to grind back for repairs.
The heat from the sun is the best way to determine how well a repair is going to hold up. Setting a repaired rubber bumper out in the sun for a couple of days before you paint it will tell you if the repairs will hold up after painting
After grinding out the repair spot, blow out the dust, apply a fast evaporating wax and grease remover. Blow try again, then apply a adhesion promoter. You can find it in a auto body supply store. Apply the repair material trying to prevent any air pockets. After sanding, blow it off real good, wax and grease remover again, apply adhesive promoter, then prime. If possible after primer air drys set the bumper out in the sun a day or two. Any problems in the repairs will show up then.