First of make sure you didn't get dirt on the pad when you glued on the key. Second glue alone isn't always the best way so try other options like make small hole for a pin to fit and hold it better in place if possible.
If it doesn't work then one might be tempted to take a key from an old A500 keyboard but they aren't always 100% alike in colour but if you can live with that then there shoudln't be a problem. I did this once and found that the keyboards don't use the same "springback" system and in my case the F3 key would stand out further than the other keys but I could just switch between it with the one under Esc so the F3 got in same hight as the other F keys and instead the Esc key was a bit higher, less notisable though. :-D
