The rivets tend to lossen w/ miles.Try drilling the old out .Iinstall a new one with some STRUCTURAL silicon....works a bit better than the reg stuff but the other stuff is OK.You may also want to drill a few extra holes and install Black plastic license plate screws.
If you dont have the rivet gun for the cat rivets try 2 pair of pliers .One to hold the rivet in place and one to grasp and lever(off the other pliers) the pin out till it snaps.