Andersen latch or mortise lock loose

by Emily
(Delaware)

Andersen 2565507 mortised lock (discontinued...of course) with Thumb latch on patio door. Latch is so loose, it falls locked on it's own. No visible way to tighten either part. When taken apart, lock turns with no resistance and latch falls without resistance. Which part is the problem? Mortised lock isn't sold separately, so do I need new combo? Thank you.