The spark plug cap "screws" into the plug cord. It is not meant to just be pushed on.
Pull it off again, check the condition of the core (wire). If you can only see two or three strands of wire you may want to clip off a half inch of the then to get back to the meaty wire. DON'T clip too much though or you will not have enough cord to reach the spark plug when you put it back on!
After clipping, "screw" the plug cap back in in a clockwise direction and then attach a small ziptie over the rubber boot (to keep it from pulling out again).
Hope this helps.