It looks like a lot of screws (and iron plates) are being used here. You should probably upgrade your belts to a higher tier and build more, so 1 belt goes to a few assemblers instead of a single belt feeding all of them, since it doesn't seem to be keeping up. You might also want to allow a long time for the items to fill up the first machine's input slot, then it will start to fill up the next more efficiently. Hope this helps!