Does your pins on your quick tach go down very far into the latch area? I have a 1 inch pin that is tappered and it only goes down into latch so about 1/8" to 1/4" of the pins thickness of the 1 inch pin is holding on all 3 attachments. I was concerned about this from the beggining but told that was the way they hooked up. I am not sure that the bottom of the QA seats into the plates to allow the pins to fully catch. Meaning as you attach the top goes in and you roll the bucket back. the top is real close but he bottom has about 1/4" or more gap between QA and plate on attachment.
The first day I hooked on my grapple and rolled the bucket forward or down The pin slipped out of right side and dropped the grapple. I had 3/8" plate welded on both sides where pins go into grapple, seems to helped, but still obviously not correct because there's a 1/2" gap on right and none on left (where pins go down into grapple.)
Now I'm having the same problem with bucket not fitting right, I can weld plates on it too, but there's a problem somewhere either in the loader or the QA.
There are variations from every manufacturer of the quick attach plates. For part of the problem I would wonder if your quick attach might have gotten tweaked. Yes some are just that way, do be sure they are moving freely and the maximum distance as your problem might just get worse!
When I changed my fel over to my Mahindra QA and matching 6' bucket I had the same problem with pin engagement. I bought the carrier & bucket as a set from a Mahindra dealer but was made by KMW, the brand of my fel. When I would back-drag with the bucket it would remove it from the tool carrier. Sometimes the bucket would bounce off if going too fast on some rough ground too. Luckily both the carrier and the bucket had some flat metal area that mated and I drilled a hole in each side and I put a hard 5/16" bolt thru it as a safety for the bucket coming off. That has worked for me now about 12 years.