Colorado T Bird,
Remove the deck shelf receptacle and check its condition. Reach in under the shelf and remove the nut that secures it to the rear deck. Try locking it onto the pin to check for burrs or worn recesses that the side pins lock into.
If its not the culprit, the pin or the spring that keeps it in the locked position may be. As paul2748 has already suggested, lengthening or shortening the pin on its screw shaft may be the fix.
I've been there with the same issue and it was a fiddly issue for sure. Good Luck!