AT HIGH RPM THE BOAT HAS NO SPEED
First off, all props slip--as much as 40-percent! They are supposed to! For a complete treatise on how a prop works, check out Dave Gerr's excellent book, "The Propeller Handbook."
Back to your question: I suppose you mean you think the prop is spinning without doing anything to create thrust. I'll assume that suddenly your engine revs up to a given RPM, but you get a less-than-expected amount of motion out of the boat. When this happens, it usually means that you have spun the hub of the prop. The hub is a polymer/rubber designed to break free should you strike a solid object underway. In so doing, the hub helps to protect the gears.
Now, while these are designed to break away when a strike occurs, they also just naturally wear out. In any event, bring your prop to a prop shop and they can re-hub it for you.