Try refreshing the game. If you're playing the Flash version, try switching to WebGL or vice versa.
Otherwise, contact SUPPORT: https://portal.pixelfederation.com/en/diggysadventure/support
Scroll down for category options to begin the form.
PS: the keys count as tiles, so if you got the key from THIS location, then you're missing a DIFFERENT key. They're all the same key, so it doesn't matter which you've already used. Futuro Forest, you had to roll the 6 stone into the pink scarf to get access to that location's key.