you could always try soaking it in penetrating spray which might help it, if the bolt is already in then try looking at the end of the bolt if it sticks out and see if you can find the park what is not right on the thread, other thing you could try is to heat up the area and it might make it come out easier as it should expand the metal, but if your taking the bolt out then it should come out as the rest of the thread should cut the line ok. only thing i can think of is that you either got some crap in there when you put the bolt in last and it has made it stick the way it is.
I would try spraying as much penetrating spray in the thread line as you can get and then loosen the bolt till it stops then spray more up the hole then keep tightening and loosening it as the lubricant should work its way up the thread and make it easier.
Failing that heat it up till it is glowing then try undoing it.