Mortise locks are commonly found in commercial and residential buildings. Many mortise locks are installed with a trim plate on the door that has predrilled holes for the lock cylinder and door knob. Keep in mind that these are trim plates, not cylinder guards. These trim plates are hollow and can easily be dented in to allow the lock pulling tool a better bite on the cylinder.
After verifying that the door is in fact locked, dent the cylinder guard in with a tool to expose the sides of the cylinder. Once this has been done, set the lock puller onto the cylinder and give it a few hits to set it in place. Now pull the cylinder. With this type of lock it is sometimes helpful to rock the lock pulling tool back and forth to loosen the cylinder up. After the cylinder has been removed, you can proceed with your through the lock operation.
This video demonstrates rocking the tool from side to side to assist in removing the cylinder. Be sure to always have your irons available in case the through the lock operation fails as in this video.