Unable to modify items in item specific guis using procedures

I am attempting to make a procedure that should shoot an item in the inventory of a specific item, and remove it from the inventory. The code detects that an item is there, but is not able to modify it in any way. I tested this with many blocks that modify item stacks, including: Enchant itemstack, Shrink Itemstack for X, and set number if items in itemstack to X. I also tried this without the variable, but it still resulted in no change. Here is a screenshot of the part of the code that does not work.