Sometimes when washing materials such as gravel or sand on a belt, it gets stuck, never finishes washing it and has to be removed manually. In the case of gravel, removing it once is enough for it to work, but in the case of sand, after the fourth or third wash, it fails.

From the video it looks like you have a TechReborn Crude Storage Unit connected directly to the belt. This is known to cause all kinds of issues, for example dupes. It is probably also responsible for the bug you're witnessing. Try putting a Funnel and chest between the belt and the Storage Unit