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
They only suck up things every 7 ticks. If you want them to suck things up, make sure they move slowly.
It has been changed to 4 ticks in the next snapshot.