Same thing happened with copies of FE7 too, IIRC.
If you run into this, it's unfortunately a different problem. Fire Emblem 7 doesn't use a save battery. I checked inside my cartridge sometime earlier this year to confirm it.
Huh, I always thought that GBA cartridges used batteries to save games...
Some do. My guess concerning Fire Emblem cartridges losing the ability to save is this: cartridges that utilize flash save usually last for about ten thousand writes. Because Fire Emblem autosaves after literally every movement, it runs through writes faster than other games.
Is there anyway to fix it with FE7? does it happen with FE8 too?
You can do the following:
Get a new copy of the game. This is inadvisable, as you'll most likely be purchasing a used version of the game, and you don't know how many writes are on it.
Get a flashcart/flashcard.
Emulate.