Once the mechanism gets worn, there's no fixing it except to replace the whole lock.
But before you do that, see if it's the lock itself or the key that is worn. If you have a spare key, compare the two keys and see if one is worn down more than the other. The key is made of softer metal than the lock and should wear faster than the lock tumblers. The fix might be as simple as using a new key.
Engineering: Finding complex solutions to simple problems you didn't even know you had.