I learned that on our '76 tril it works best if I back the wing nuts off until there is no thread showing, then press in. You'll hear a 'click' and notice the frame of the rock guard move a little. You're there. I also find them a little tricky to engage.
Don't know how old your trailer is, but I found the corrugated fiberglass panels in the rock guard to be quite deteriorated - to the point where the little glass strands were sticking out - not good. I replaced them with something else but with hind sight might have tried to refurb them with several coats of polyester resin and some paint