I apologize, for some reason this game eats save slots. I know that's bad but I have tried to fix it and for some reason it seems to happen in some other games created with AXMA 6 as well. Unfortunately the game creation software isn't really supported anymore. It uses your browser cache to save, so if you can selectively clear cookies (if you dig in you only need to clear for this game/url/website if you don't want to clear all of your cookies) it will clear the save slots and you can use them again. Alternately save again on a new slot and make sure you don't try to restore it on the game over page which seems to be what corrupts each slot selectively. Another option - re-play in a different browser which will access a clean cache for it.
Since the software isn't supported and updated anymore for about four years now it's possible there have been browser updates that don't work well with AXMA and may be exacerbating the issue.
NOTE: I've noticed that attempting to restore from a save slot on the final "game over" screen seems to corrupt that save slot. If you're at the end title screen with credits, please restart the game before restoring so you don't lose a save.