Maybe its not the same version of the game.
Mostly, RenPy can only loads save data that made by the same game version. If you update the game version (which you do to get the latest chapters), it's not guaranteed the old ones can be loaded. It's a function the dev has to implement carefully and only few devs get this done without any problems.
[Little Spoiler of "DDLC" and" Historia"]:
Other games, like "DDLC" or "Historia" have a intended self-destroying mechanism that destroys your progress when you hit a bad end, but in this game it shouldn't be the case.