Minecart with hopper dissapearing

I have made a farm in MC:PE that traps chickens and makes them lay eggs. There are iron bars to surround the contraption so the chickens can't escape. Then, the floors are made of blocks of hay. Underneath the blocks, there are mine carts with hoppers that move back and forth on powered rails. Underneath the powered rails are hoppers that lead to a chest. The problem is, though, that whenever I leave the area for a while, I notice that the eggs don't get collected and are stuck on the hay. When I look under the hay, I don't see the hoppers. They disappear randomly and it's very annoying because I'm playing in survival and the mine carts carts are hard to make and it's a very big waste if they just disappear randomly. Please fix this, it would be very appreciated.