When you try to throw an item into a a hopper with all item slots filed, it won't except anymore items

I built an automatic sorter with the new hopper blocks, and to do this i fill all item slots with diamond for example (but i don't fill each slot with 64, i just put at least one item in each slot) but when i try to throw another diamond in the hopper, it will not take the diamond even though it has room. This is relevant to all metal items (don't know about others). I was in a superflat world with the preset overworld minus all attributes (biome_1...) except for village i left on.