Patio door multi-point mortise lock with pocket plate (12")

Thanks for posting. Sorry to hear of your trouble. I agree, your mortise lock looks like the one you purchased (our 82-260) and so it's not a matter of the incorrect part. If you try manually throwing the new mortise lock with a slotted screwdriver, does it fully engage? If not, then it's possible there's something wrong with the new lock we sent you, and we'll be sure to send you a new one. If the mortise lock engages just fine manually, however, then I'd like to see a closer photo of the tailpiece on your old thumb lever--perhaps it is damaged or warped. Many thanks. 

I think the issue may still be with the mortise. Based on everything you've described so far, it sounds like a faulty spring. I definitely don't think it's the thumb lever; looking at your photos, I see no damage that would suggest that it's broken. 

It's easy enough to eliminate that as a possibility, anyway. Simply remove the handle from the door and flip the lever. If it moves without issue, then you can rule it out as a culprit. While you're at it, you should still try the screwdriver trick that I suggested above.