Same question as [MENTION=43936]zcruiser[/MENTION] asked: have the holes in the flairs (bolts go through the holes in the flairs to the body) been enlarged in anyway?
As long as the bolts aren't stripped or missing you can you can probably use fender washers between the bolts and flairs to hold them down.
If your flairs have been distorted in anyway, as in they no longer follow the contour of the fenders, then you may have to reform them carefully with heat or replace them