minecarts with hoppers often fail to suck up items, even when they are not moving

As the title says: minecarts with hoppers often fail to suck up items, even when they are not moving.

steps to reproduce:
1. make a track 5/6 blocks
2. put a minecart with hopper on one side
3. throw some redstone dust on the tracks on random spots
4. give the cart a small push and notice how it fails to suck up all the items