WD40, liquid wrench, or something that will loosen it up. Tighten all the other bolts down as hard as you can then try to loosen that one. You should have any problems rounding it out if you have the right allen and it's in there right.
the same thing happened to me. i dont know why it does that but u just have to hammer the allen key in farther and it will loosen and if that doesnt work for ya then i dont know what to tell u.