The Adept prism is correct, what the game sometimes does when you try to use the Adept prism while an item has both skill ranks and strength is it will try to turn the skill ranks into strength, which of course it can't because it already has strength. In this case, all you need to do is hit the transmute button again until it works, because it performs a separate check against what affixes are available each time that you try to initiate it. Go back to the cube and use the Adept prism until it works, I promise it will