If you have a water stream running over hoppers, there seems to be a chance that the hoppers will not pick up the items that are in the water stream. I tested this several times, with multiple setups, and every single time, there were a couple of blocks that would "skip" over the hoppers.

To replicate the issue;

1 - Place a hopper and 7 blocks in any order (hopper can be anywhere in the lineup)

2- Place walls and put water at one end so it runs over all the blocks

3- Drop blocks into the water stream (before the hopper)

Over maybe 20 tests of 1 stack of items each time, I found that anywhere from 8 to 29 items would not be picked up by the hopper at all, and you can even see the items just float right on over. I tested having the hopper at the very end of the stream (so the water is as shallow as possible) in case the items were "too high" in the water, and some still skipped over it or didn't get picked up. I also tested this both in realms and in single player; the issue persisted.

With more hoppers (eg a lineup of 3) the chance of one not being picked up by any of them is very small, but it can still happen. This problem mainly affects storage systems, in which most items only have one hopper to be picked up by. If there is a chance that upwards of 30% of your items don't get picked up, that can cause some real issues.