It seems items with a higher quality than normal get stuck on belts, the machines will not use them even if I tweak filters on the inserters, we have had to filter them out to keep the machines going, most noticeable on cogs and wires/plates.
Honey, that's base feature introduced in 2.0, if you don't want it disable Quality module that came with the base game, otherwise you need to specify quality in filter/recipe
