I've got a sugar cane farm in which I use cart with hopper. The problem is that the cart with hopper disapperas everytime, so it's consumating material (most iron) and it's useless. I don't know if it matters that the cart is moving or not but carts with chests that don't move most of time don't disappear. They don't move but they maybe don't disappear 'cause they're with chests a maybe the problem is just with carts with hopper (that's what I think). Please try to fix it.