Do you have a door that is difficult to lock from one side? Mortice locks are normally designed to operate from both sides. If key is mostly used from one particular side of the door the key can wear unevenly.
An indicator of a worn key is it working better from one side, and/or having to jiggle the key up and down to get it to turn. Getting a new key cut from an original (not a copy) may solve your problem.
In the example above key has worn but this made worse by being a badly copied key from a high street shop. Points A and B should be flat across the top but you can see the uneven wear. Because point A is too narrow it has been able to slip off lever 5 in the lock. Unfortunately the worn keys had cause wear to the levers so it was necessary to replace the levers too.