I had the same problem with the allen bolt being frozen and the allen wrench opening was stripped. Havent fixed it yet, I just noticed it when I was loosening and retightning the bolts to see if I could locate a rattle. Have to go to Home Depot and get the hex head bolts once I get the bolt out.
I did a search about a month ago and several people reported the same problem. I cant find the thread but the reason was because the top part of the hole where the bolt goes into is uncovered and junk gets in there and freezes up the threads. You should put wd40 on the top area and let it run down to clear out the junk and then make sure the threads are clean. People then put caulk or silicone over the top part to stop junk from getting back in there again.
Here are the links
Undertray bolts... a suggestion
moremonkey.com • View topic - Stuck Diffuser / Engine Tray Bolt Issue...FIX