hopper does not pick up all items dropped into it

Items dropped on a hopper don't always fall in. Where it's most noticeable is using water streams to collect items and move them to the hopper, but it seems to happen any time an item lands on the edge of the hopper block.

Included are screen shots of two non-working setups. The one where the water flows onto the hopper at least occasionally catches them if they cross dead center, but the other almost never works.