May sound silly, but check to make sure you are trying to loosen the right bolt.
Had a member in another group that reported a "spinning oil cap" only to find out they had loosened the wrong one and the reason it was spinning was because there was a nut on the back holding it in place. Make sure you tighten up the one you loosened if that is the case.
Also is it possible you have a magnetic oil plug?
Patriot Guard Rider
2000 VN750 Sere (Serendipity)
1990 GL1500 (Ole Blue)
1986 VN750 EVie (project bike, heavy custom)
VROC # 11628 / 25000-H
ARVROC # 12 Coordinator and Crowd Control
OKVROC # 18 (H)
TNVROC # 45 (H)