If your key outputs several different characters with one press, or just generally doesn’t work well, this should help.
The most common cause of keys not working consistently or at all, especially with an older board, is hair, dust, and debris. Keyboards are very good at catching and trapping these things, and if they make their way into the socket of a key, then it may start to not work well.
To clean a key, first remove the keycap and the switch. If you have an older ErgoDox EZ made before March of 2018, then you will not have hotswap sockets to easily remove the switch. Fortunately, this also means it is less likely that your problem has to do with a dirty socket, so you can skip this step.
When you remove a switch from your board, you may not necessarily see any debris, but it is worth cleaning anyway. Compressed air is very useful for spraying into the sockets and making sure they are free of anything trapped in them. If you don’t have any compressed air, simply shaking the board is a decent alternative.
If a key is outputting several characters at once, particularly in a single row or column, it may be because debris or moisture is shorting (forming a connection between) parts of the PCB that it shouldn’t. This may require more extensive cleaning and time to dry, but it’s usually not a permanent problem once cleaned.
Also, double-check that your switch doesn’t have a bent pin. Occasionally, bent pins will still allow a switch to sort of work, but they may cause other strange behavior when hit.