Hoppers not taking items properly

I have built a item sorter and I have a stack of 17 of the same item in a hopper and the rest of those slots filled with the same item. When the item its supposed to suck in from the hopper above this one will just pass right over it. This will only do it for some of them though like for me hoppers, iron ingots, emerald ore, enchantment tables, and a few others.