A lock should operate smoothly without requiring the key or handle to be forced. A key that has become stiff, will not insert fully or only works at a particular angle, may indicate dirt, debris, cylinder wear or an internal obstruction. Trying a spare key can help establish whether the problem is caused by a worn key rather than the lock.
Loose handles, grinding sounds, repeated jamming and difficulty shutting the door should also be investigated. Regular maintenance and inspection can help prevent minor lock problems from causing a complete failure. Avoid forcing a stuck key or dismantling a faulty mechanism, as this can increase the damage and may leave the door impossible to secure.