Hacking Everdrive GBA x5 Mini - Pokemon games save once - then not again?

[Bronsonator]

My Everdrive works perfectly - boots, plays, and saves games no problem.

However, all gen 3 Pokemon games fail to save after the first save. I can start the game, play for a bit and save. Reboot the game, save loads fine. But when I go to save again after more progress, it fails, giving a corruption notice, and reverts back to initial save. Any idea why?

I've tried multiple versions / copies of the roms, and the result is the same for all of them.