Yeah that's definitely something we wish we sorted out. We initially were going to make a PlantsVsZombies style game where enemies could destroy the instruments and the "vanishing if not on the board" is a remnant of that.
Completely agree, we definitely need to make the game's objective clearer (and make a tutorial, proper levels, etc, etc lol).
Regardless, thanks for checking our game out! This is our first time making a game where we both kinda knew how to use a game engine and we had a lot of fun even if our game isn't the best (or balanced lol).