It sounds like you've got a rusted or broken-off stud. A little Kroil and some heat, plus a pair of vise grips will probably let you unscrew it, but don't break it off flush with the manifold or you'll have a devil of a time getting the rest of it out. Some of the older intake manifolds had a heat riser passage that exposed two carb mounting studs to exhaust gas, and they rotted off pretty regularly.
Jerry
