Mortice locks can become difficult to open for various reasons. Examples are a worn key, misaligned door, worn lock, or a foreign object.
This British Standard 5 lever lock has just been removed from a steel welfare toilet unit. Customer could not unlock the door. The original keys were in good condition. The door could not be picked and it didn’t feel right when trying. This tells a locksmith that the lock is faulty.
The only option to open the door was to make a small and accurately placed hole through both the door and the armour plate of the lock. Once the lock was disabled the door could be opened.
You can see the gap between the side plate and the body of the lock. A lever spring is stuck in the gap. This allows the lever to slip off the key and not spring back. This is quite often seen with this make and model of lock. If I fit them I will Loctite the screws in place to prevent it happening.