Stiff door lock – key turns with difficulty

The most common cause is lack of lubrication or a slightly warped door frame. Spray a lock-specific lubricant (e.g. graphite spray or PTFE spray – avoid WD-40 as it attracts dirt) into the lock cylinder and turn the key several times. Check whether the door rubs when closing – this can be caused by a misaligned strike plate, which you can adjust with a screwdriver. If the lock remains very stiff, have a locksmith inspect it before it fails completely.