It sounds like either the bolt (stud) or the nut is stripped. Either way if you want to replace the stud you can. The suspect stud can be taken off, generally, with an allen wrench. Check the end of the stud to see if has an allen head. If it doesn't you can just grip it with a pair of locking pliers and back it out. You could also consider just replacing them all. A new set should have allen heads on the studs and that is how you install them with some blue Loctitie also. Make sure you also use new copper washers, don't reuse the old ones. Also, make sure the Loctite has dried thoroughly and that the stud is inserted far enough so the acorn nut doesn't bottom out before getting tight enough.