Painting Rear Bumpers

I coated my front grill, emblem, with PlastiDip spray paint. All good thus far. A few weeks later I did the same with my rear bumpers -but- the paint comes off too easily as it's more exposed.

I do not remember hitting anything (someone could have possibly bumped me parallel parking). It also looks as if debris is chipping it off or something else is causing it to breach and peal. I have to keep retouching it.

Any solutions? I was unable to take the bumpers off (to much of a pain) to paint but it worked out pretty well.

Is there another paint that would better suited for rear bumpers?

 

Tacomabeast sells some abs plastic caps that are paintable and look very easy to install over your rear bumper