patio door lock problem

Hi, I have a problem with a thumb turn on a sliding patio door and am wondering if I can fix this myself, and how to go about doing so. The problem is that the thumb turn (interior, lever type, with keyed lock on the exterior) is loose or jiggly, and does not properly engage the hook on the mortise lock to push it up and out properly. My lock looks like your 82-227

The lock hook does move up as the thumb turn is pushed down, but not always out far enough using the thumb turn alone to engage in the keeper in the door frame. I can manually pull out the hook, hold it in place with the thumb turn as the door is shut, and and then lock it into place using the thumb turn (the thumb turn is still loose and jiggly once the door is locked).

I can unlock the door using the thumb turn but there is too much play in the thumb turn to guarantee re-locking smoothly without having to manually pull the hook out a bit further once again (the thumb turn pushes the hook up and out a bit, then I have to pull it out a bit more in order for it to engage in the keeper).

Is a simple adjustment required or is something actually broken? (and I'll need a walk-through...) Any expert ideas would be most appreciated!

Here are a few pics of the lock with the thumb turn.
I'll send you another email with more pics showing how much play is in the thumb turn itself. Note that I CAN manipulate the hook in the mortise lock with the thumb turn, but I often have to flip the turn up and down a couple of times for it to push the hook out far enough, and it doesn't always work (i.e. I have to pull out the hook with my fingers before engaging it into the keeper).

here is the follow-up email to the last one with the rest of the photos. The last four are of the thumb turn showing the amount of play (jiggle) in it. In the first two I am showing how much lateral (side to side) movement there is (it isn't much, but you can definitely see and feel it if you hold it) In the last two photos I just let go of the thumb turn and you can see how much it dropped from the position I was holding it in (as if reading a clock - from the "2:30" little hand position to the "3:30" position). you can also see how much side to side movement there is in any position (look at the left edge of the thumb turn in the last two photos, see how it overlaps an edge in one photo and not in another). Is this looseness of the thumb turn a mortise lock problem? Or is there a small piece inside that needs to be replaced?