How to attach bumper cover so that fits evenly?

Hi. I have a 2006 Toyota Corolla, and as of about three days ago, there was no problem. Now, (not sure why); just want to fix...
the Driver's side front bumper cover is not fitting flush with the fender. There is about a 1cm gap.

I have tried completely removing and re-attaching twice with no luck.

Any suggestions? (Ie. Besides going to a junkyard and buying a new bumper cover), as this one fit fine until recently.

Please see photo's.

Thanks,
RolloAttachment 50157Attachment 50158Attachment 50159Attachment 50160

P.S. 1st two photos of passenger side, fit fine. Last two photos show gap.

Usually there is a retainer strip that is screwed to the bottom of the fender that has tangs that engage the square holes in the bumper cover. Most likely the retainer is broke as it looks like at least one hole in the bumper cover is not torn

Thinking that if you don't know when that happened someone has hit you with something while it was parked. Can't tell for sure but it almost looks like the fender is pushed in slightly.