GUI output stack

I have been making a block for my mod, its a crafting station and it works like this: you put items in one slot and a second slot you press a button wait 15 seconds and you get a item in slot 3 of the blocks GUI. The problem is that when and item is in the 3rd slot you cant get another one = they dont stack. You press the button, the items in the 2 other slots dissapear (thats supposed to happen) but a item dosent appear in the 3rd slot.