We had a peculiar mortise lock problem at our local church. For those 'down under' the lock was a new style Lockwood 3570 with usual Australian type oval cylinder and turn knob. Turn knob is part of inside escutcheon and escutcheon are held in place by top and bottom machine screws passing through door.
Turn knob would not turn without alot of fiddling. Near the end of maintenance period, builders 'fixed' it and it seemed OK.
A bit later someone complained that the escutcheons were loose, so I tightened them. Lo and behold the turn knob would not turn so I backed off the screws until it worked pending a solution. The builders had slackened off the screws slightly so it would pass muster on final inspection! I thought the tail piece might have been a bit long, but if this were so, there would be some pressure when turning the key. When I had time I very carefully checked the alignment and enlarged the screw holes and spindle holes - to no avail. I noted that the tail piece was slightly off centre because of a slight machining error on the escutcheon assembly, so I filed a piece off the tail piece to compensate - still to no avail.
A carpenter in the congregation also had a look and he suggested shortening the tail piece which I dutifully did (but see above) - still no use, and he widened the inside spindle hole further to ensure nothing was catching - still no use.
We noted that the mortise was off centre (the door thickness was about the minimum for installing such locks) and figured that handle assembly on the inside escutcheon was pressing against the lock case - this was confirmed as there was a bit of friction when turning the inside handle. It was OK with the top screw fully tight and the bottom screw slackened off a bit.
The 3570 is a standard reversible lock case with deadlatch bolt capable of the usual functions by the use of different cams and accessories. The key feature is a split latch hub with a locking bar at the rear of the case which moves up and down in response to turn-knob and / or cylinder as need be. A few adjustable parts allows this to lock one or both handles so escape, classroom functions, etc can be selected.
Now we figured that the handle assembly pressing on the lock case distorted the case slightly and caused the locking bar to bind which meant the turn knob would not turn. We will now cut and place a shim under the bottom edge of the inside escutcheon.
I would like to wring the necks of the carpenters who installed these locks! In retrospect I wished I put my foot down with the architect and insisted on Schlage AL locks, they would have been better for the thickness of inside doors specified and the duty cycle would have been OK. Some late1960's A series Schlage locks (the ones where you had to dismantle everything to get at the cylinder) had performed satisfactorily for 25 years on the building previously demolished to make way for the new building.
The old hands have no doubt been through this sort of scenario plenty of times, but perhaps a few followers of this ng may learn something useful from this.