The problem is not the key. This cap has a very tight fit and the spring mechanism which holds the gasket needs lubrication or it binds. The binding of that spring and gasket is what you are fighting.

I periodically lube mine with white lithium grease but I still work the gasket/spring mechanism up and down a few times prior to attempting to shut it. Even when the grease dries up working the spring mechanism (not the key) greatly improves your ability to close the cap.