My best guess for you:
The save battery inside the cartridge has most likely died, or is close to it. There is a small cell battery inside the cartridge which powers the memory for saves and the like. Over time, it loses its charge and, and your game won't save/saves disappear. You can replace the battery yourself, if you know what you're doing, and have the tools. I recommend going to the iFixit YouTube page and searching for the project. You can probably just do a general Google search for it and find it.