Sittable objects randomly don't work

Spell-Slinger

I've placed a bench in my house, it shows green when selected, and yet I can't actually sit on it. If I move the bench slightly to another part of the house, it works. If I then press z to undo, it jumps back to the original position and suddenly works (though only one out of four seats). When I leave the house and come back, it goes back to not working.