There is! The thing is that the keys have a 25% chance of appearing, so if you're very unlucky well... it can take a while.
The more keys you get, the more it's unlikely to find the remaining ones as you first have to be randomly spawned in the stage where the key is missing, and then you also have to do it multiple times until the key appears, so basically if you're missing just one key, you'll have to be very lucky to find the last one.