That's so strange about darker threads being more problematic! I don't really know what's going on. Machine is cleaned and oiled, i tried different needle sizes, adjusted bobbin thread and top thread, even taking apart the tension assembly to see if anything changed. I can't figure it out so i'm switching thread. I've heard that 15 machines are very forgiving so I really don't understand why 30 weight thread should be a problem. I spent an entire day on it and nothing doing. So now I'll just wait for the new thread to show up.