If all slots of a Hopper is occupied by any items the hopper won't suck up any of the containing items when thrown upon it if there already is an item within "pickup range" that doesn't have a spot inside.

In the picture, i filled the hopper with planks, redstone and stone.
Then i threw a piece of dirt on top (which of course does not get sucked up)
And then I throw one of each of the contained items, which is not sucked up even though there is applicable space inside.

It seems the hopper keeps trying too suck up the first item thrown upon it until it succeeds before testing with another item.