Caramel sauce thinned out over 24 hours

I decided to make some of these salted caramel mini-cheesecakes, and all in all, they turned out pretty ok. The problem I had is that after about 24 hours in the fridge - since I had to make them a day ahead of time - the caramel on top thinned out a bit and was sliding around instead of being a nice gooey blob on top. The 'skin' of the caramel was still firm, but it seemed to be riding on what became a layer of dissolved liquid of some kind. Right after the caramel cooled, it was perfectly solid, but after coming out of the fridge, they started sliding all over if the tray wasn't perfectly level, and made them look pretty sloppyI'm not why that happened, so I can't fix it for next time. Any ideas? via /r/Baking https://ift.tt/39JZpdW