The black plastic RAM tabs are strips of plastic, which loop around the concealed RAM release lever.
As the 20” Macs tend to get quite hot, I found the glue on one of the tabs had come loose, causing the loop to separate. I discovered this on an empty slot when doing the 2x2GB SO-DIMM upgrade - the RAM not seating properly is a bit of a giveaway, and a quick inspection with a flashlight can confirm.
If this happens, don’t panic!
BEFORE YOU INSERT THE RAM - grab your spudger, and press the tacky parts of the plastic strip back together. There should be sufficient glue to give a solid bond.