You mentioned buying a weld on adapter plate when you got the QA, but since bought a new bucket. Have you tried putting that adapter plate on the QA to see if it locks down? If it does then problem is in the new bucket and little grinding should fix it, if neither will lock down then it is in the QA and need to look at that, make sure there is a weld bead sticking out in the front that may be holding that corner of the bucket from coming in far enough.
Update
I just took the tractor over to a tree stump that I haven't cut down and pressed the bucket up to it and the pin fully latched down. It would stink to do that every time I want to put the bucket back on...better not cut the stump down any time soon.