old mortise lock issue

Hi I replaced my mortise lock with a new system. I didn't replace the guts inside the door, since that was another $400 to purchase. I just replaced all the outside fittings. I purchased exactly the same brand and system that I had before so it would all fit. So far so good. Everything back in works except that the door knob keeps slipping out and the half spindle comes right out of the door (at least half of it does)...how can I tighten that so it doesn't come out?? THANKS!!