Hopper Minecarts Ignore Items

I have an item sorter/ double speed shulker box loader with hopper minecarts one block below an item water stream ( I tried ice and less slick blocks) and occasionally (About every 1-3 batches of items) it's skips even though it hasn't had any activity for 8 ticks. The item amounts are from 6-10 and the Minecarts have plenty of room. The batches contain about 4 item types of about 20 items. I just build this in this beta.