[SOLVED] GUI not accepting items in input slots

I am trying to make a block that contains a GUI which accepts an item and a bunch of a certain item as well as another item and produces a duplicate of the item. The GUI has 10 slots properly IDed 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. Slots 1 through 7 accept the "fuel" items, slot 8 is the input, and slot 9 is the output.

All of the input slots will not accept items. In the test world, when I attempt to use the block, the game refuses to allow me to put the items into the input slots. (Think of what happens when you try to put something in an output slot; it behaves like that) All of the "filter" settings are unset.

Anyone know what could be going on?

Edit: I did some experiments, and found that hoppers will insert items into the OUTPUT slot. They will not insert into any other slot. I have absolutely no idea why.

Edit: Issue solved: Apparently the settings on the block for preventing hoppers from interacting with a block's inventory also happen to apply to players. I am still not quite sure why hoppers were inserting into the output over all the other slots, but that issue is now irrelevant.