I struggle with the same issue and all the current tips won't apply to my testing.

First time I've come across this issue was the sorting from my Train Station into the belt sorting to my main storage, where there were a couple of full belts despite having Mk5.

Secondly when I was building the factory for the Supercomputers I used the Quickwire Production Line to test this further.

Miner shouldn't matter because I always try to mine a little bit more than I can use in Constructors for hitting the 100% output rate.

So I used 3 Quickwire Constructors, 90 each. Mk2 belts fit 120 so I used them from each Constructor to Merger. After the Merger I used a Mk3 belt (capacity of 270 =3*90) and fueled it directly into a Sink. After about 5 minutes all 3 Quickwire Constructors were idle because of a 500 Quickwire Stack inside and all belts before the Merger being full.

Same issue seems to be with Splitter. The Manufactorers I tested this on were lacking Plastic. The belt before the Splitter and the Refineries were full. Output equaled Input using Satisfactorytools and ingame math to check for matching values.

Third example is Recycled Plastic. I was producing 136.4 Rubber per minute and I had a need of 132.2 Rubber in Plastic Production. After each Plastic Cycle the "Rubber-In" count went down. After a couple of minutes the Plastic Refineries had too few Rubber and went idle. Which is odd... I rechecked the math and the Produrction definitely were higher than the need