Hopper don't suck in what they actually can suck, when something they cannot suck were dropped on it before. I cannot describe it very clear but I know how to reproduce it.

1.Set a hopper on the ground

2.Place 5 item in it, occupy all 5 seats, take crafting table for example.

3.Drop something else, take a slime ball for example.

4.Drop more craftting table but they cannot be sucked in.

5.Take away the slime ball, then crafting tables will be sucked in.

From my friends I get that this bug have existed for many versions. It has affected many sorting machines. 

After my testing in java edition, certainly this bug only exist on BE platform