Fortunately the pieces you are talking about are not expensive.
I'd take that head off... then try to clean and grease the lug. Take a different screw and see if it works out fine. Inspect the other screw and check to see if the threads are worn.
Depending upon the lug or screws the parts should be less than like $10. Depending on where you go they might even have spares that they throw at you just to try to keep you coming in.
