Do you have a mortice lock that is difficult to lock from one side? Mortice locks are normally designed to operate from both sides. If the door is mostly locked/unlocked from one particular side of the door the key can wear unevenly.
If the key works well from one side of the door but not the other it may be as simple as a the key has worn. Getting a new key cut from an original (not a copy) may solve your problem.
In the example above key has worn but this was exacerbated by being a badly cut copy 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 parts in the lock.