that inner retainer might be broken or the bumper tab itself, sometimes the plastic part that holds the bumper to the fender needs to be replace and sometimes the fender itself will need to be replaced because it is worn out from pulling it off and on. I will say most likely all you need is that plastic part that holds the bumper to the fender..

 

mine had the same problem. it had been in for the front bumper replacement. they never could get it to fit properly. there is a little tab that is part of the bumper. If it has broken off there is no way to re attached which the body shop did. only solution is to replace the bumper cover. rather cheap construction for such a heafty looking bumper

 

I once had a problem like this and was able to repair it with "super glue". If the only other option you have left is a new bumper skin you don't have much to lose. I don't know how long my repair lasted thought because it was a lease return. It looked good when I returned the car.

Dave Mac

 

This is more common on the rear bumper cover - usually the clip, so I suspect its the same on the front bumper.