Stub Mandrel Now is your chance to become a 'lock smith'. If you have two keys check them for wear, first by eye and second by measuring. Remember that locks are moderately precision bits of work. In a lever lock there are two bits which protect the bolt. The first are the 'baffles' which take the form of cylinders which if the key does not have corresponding groves in it you cannot turn the key, The second part of the protection are the 'levers'.these have narrow cutouts on them. Lift them too high or not high enough and they stop the bolt from moving..
What can go wrong? Broken springs on the levers, wear causing ridges inside the levers, damaged baffles. wear to the keys and the key axle bearing scuffing in the sliding surfaces, lack of luberication etc.
To cure dismantle the lock carefully watch out for the springs. I always number the levers so i can put them back in the right orders and when you do lubricate them well. Get rid of burs and straighten up baffles etc.
Locks are fascinating things designed to work for years with complete neglect. Clearances are quite large to allow for wear.
PS how do I know My house builders fitted the cheapest of the cheap locks so I often have to attend to then to get some one out of the 'kahazi' and then re-engineer the door
Good luck
Rdgs
Dick