Skill issue :P
Edit: to try and actually be helpful, keep in mind that this game is inspired by Darkest Dungeon: a lot of things can fuck you over, and sometimes it's easier to get rid of a hero who isn't pulling their weight. Get rid of items you know are either cursed or replaceable when you're in the dungeon to free up room, and accept that sometimes the box shouldn't be messed with if you aren't using expendible heroes. Also, surrender is always an option: if you value your heroes over your loot, you can always run away before they get cursed/kidnapped. Hopefully this helps you enjoy the game better :)