Many times a door can not be locked because the mortise lock is broken.
You must replace the mortise lock with the same version (you can determine this by color of the cylinder Gray or Black. Or by the angle of the cylinder Gray is at a 45 degree angle where black is at a 90 degree angle) If you do not choose the correct one keyed cylinder will not fit/operate.
If your Cylinder is Sliver - then choose the gray as it as the 45 degree slot
Use Keeper Part # 533 with this mortise lock and Keeper Finder Part # 536 with Keeper.