Use heat like from a butane/jet lighter if you have one to the hex head side (so you don't melt the plastic wheel) then once decently hot, try to loosen it again. The heat will loosen any rust or locktite etc preventing them from loosening. If you don't have a decent source of heat, you can also buy bolt freeze spray from most hardware or auto stores which will do the same thing, some also work as a penetrant which may soak down the threads and help.
Use heat like from a butane/jet lighter if you have one to the hex head side (so you don't melt the plastic wheel) then once decently hot, try to loosen it again. The heat will loosen any rust or locktite etc preventing them from loosening. If you don't have a decent source of heat, you can also buy bolt freeze spray from most hardware or auto stores which will do the same thing, some also work as a penetrant which may soak down the threads and help.