1996 Plymouth Voyager drain plug for radiator

Some plugs have a sort of anchor attached to the water side and tabs inside to prevent them from dropping out. If that's the obstruction, you may need to jiggle it a bit as you insert it. If the problem is that the O-ring on the edge is holding it out from threading, a little firm pressure as you turn it should help. It may help to hold the tab sticking down with a pair of vice grips for better purchase.