I've been thinking about tightening the truss rod of my MIM Fender standard Jazz bass. According to Gary Willis on his website, if you lower the height of your strings and the frets above the 12th fret rattle and buzz, then you need to tighten your truss rod. Well, unless I keep my strings really high, I get buzzing and rattling above the 12th fret. I adjust the truss rod once before a couple of weeks ago with a 5 mm allen wrench, though it was hard to get the wrench to fit snugly enough to turn. After looking at the screw with a flashlight, it looks a little worn, but still vaguely hexagonal. I haven't been able to get the rod to turn this time at all, either tighter or looser. Do I need a different sized allen wrench than 5 mm, or is this some problem with my screw? Is there anything else I can do?