RE: An oddd request
well.....
two things to look for....
1) see if the head is loose - you sorta noted this, but is the entire had button turning? If it is turning, turn the engine to bottom of the stroke, remove the exhaust, loosen the head bolts, look into the exhaust to ensure that the sleeve is aligned in the crankcase, and then tighten the head clamp (fins) back down.
2) If the plug just turns in the mounting hole ... the threads stripped out or wore out from a lose plug shaking. In which case you will need a replacement head button.
Loose head bolts and loose plug usually come from the engine getting hot. Be careful.