Very Strange Problem With Save on Pokemon Diamond on M3 Simply

I've been having a weird problem with my save game for Pokemon Diamond on the M3 Simply.

The game seemed to be fine up until this morning. It played fine and I saved regularly, and I saved before I switched off. When I switched back on, my save game had been rolled back to the one that I had saved last night.

Thinking it was a one time problem, I continued from where I was, and saved regularly, and everything seemed to be fine. I saved and switched off about an hour ago, but when I switched back on, my save game had been rolled back again to one from earlier this afternoon. It's clear this isn't a one time issue.

Has anyone had a similar issue to this with their saves on this game, or indeed on any other game for the M3. If so, then what can be done to fix it, if anything.

Thanks to anyone who can help.