block inventory only using first slot

I'm currently trying to copy items from the player's inventory to the block's inventory, but whatever I do it seems to only work for the block's first slot, even when manually changing the number of the slot to add the item, it only works with slot 0. 

For example, when I want to move slot 0 of the player's inventory to slot 0 of the block it works fine. But as soon as I change those 2 values to any other number it won't do it. Am I missing something obvious here?