Hoppers not taking items off blocks above hoppers

What I expected to happen was...:
To have my sheep's wool go into hoppers to get sorted

What actually happened was...:
Wool just sat on the grass not getting picked up by hoppers below

Steps to Reproduce:
1. Put a hopper underneath a block
2. Drop an item on top the block with the hopper underneath
3. Check hopper for items or see if it sits there.