A trader with instructions to keep several items resupplied will not cycle between them while one of them is yet to be filled. This can result in the trader running back and forth to a mine, for instance, to grab just one piece of gold ore while other items he needs to get are stacking up and can be transported more efficiently in larger numbers. This also blocks gold ingots from being transferred giving you buying power. In my opinion, the gold ingots needs to be a top priority above all else. Then when has multiple items to restock, he should cycle between them even if the item he is stocking is not full to the requested level yet. This cycle should take place as soon as he drops off material so while running to get item B and C, item A can continue to be produced and he’ll be able to pick up more of them when he cycles back.
My dumb trader was just running back and forth to the same mine grabbing one ore at a time and never transferring the gold ingots I requested or moving on to other products.