These have a slot in them, right? I have always used the butt end of my Sugino chainring bolt tool to remove and install this type of cap, but any thin tool that fits the slot would work. Generally they should be no trouble screwing in, but since they are plastic it is easy for the ham-handed to cross thread the caps. If this is your problem, you may be able to clean the threads but if no success, just leave them off.