To add to this comment: If after ‘reattaching’ the key it doesn’t pop up. The ‘hooks’ on the back of the key might be bent from trying to reattach.
I had this exact issue with my ‘Control’ key and only after using an otoscope found that they were bent. After using a very thin screwdriver bit (ifixit #1) the hooks were pried back into shape and the key was able to slide back into the slots on the mechanism.
The YouTube video helped me immensely (thanks Ian), for other people with a similar issue where replacing the key doesn’t work…
My ‘Control’ key was stuck flat. It seems the issue was the key itself where the ‘slot’s on the top underside of the key where the mechanism slots into were bent from trying to put it back (I used an otoscope to find this out!). I used the thinnest iFixit screwdriver bit (#1) to lever the ‘slot’ on the key back into shape so the mechanism could fit into it rather than be forced to the side.
Then inserting the key as per the video worked first time. Thanks!