Can you take a picture to show exactly how it's "coming loose" now on the car?
The fender trim is held on with D161 sized clips, and if the trim is pulled out the clip may still be in the fender. What you'll need to do is pull the trim off a bit more in order to reach in there to pop the clip out from the fender, then place it back into the trim slot where it goes and then pop it back onto the car.