I'm late to this but, since others' late comments were very helpful to me, I'm also adding a late tip, in case someone could benefit. H/t to gilly dekel who helped me realize that the bottom hooks on my spacebar keycap had closed. The "c" clips had "squished" so they were more like an "o," and could not hook onto the bottom bars. (I'm surprised that, in all the posts about replacing keys that I slogged through before I got here, I did not see this potential problem discussed. It seems likely that it is relatively common occurrence.)
To fix the clips, I used a flat eyeglass screwdriver (and a magnifying glass) and slide the tip under the hooks then very gently pushed each "o" back into a "c." Keycap popped right in.