OK, first thing's first, what OS does your computer use? Windows? MacOS? Linux?
And to that end, you've made sure to download the right version of the game for your OS?
As far as we can tell there's no specific issue where you're losing your saves. If you moved the game folders around or maybe even left the game running for a long time... or maybe just something odd happened and your saves got corrupted.
What we'd recommend, after checking your OS, would be to download the game again, extract it, run it fresh and try saving your game somewhere near the start. Close it, open it again, check if your saves are there.
If it works, then try dragging ALL the contents of the older game save folder (the one where they're having issues with) into the new download, and see if you can still make new saves.