The bolts go into a brass nut that is part of the valve cover. If one is spinning and not coming out leave it alone because the brass nut has lost contact with the valve cover. You will never get it out unless you cut it off. Trust me I know after haveing to buy a new / used valve cover.
The brass nut is sunk into the plastic with heat and over time or over tightening the connection breaks loose and the nutt and bolt just spin.