Typically, if you have bird's nest on the bottom, it is probably the top threading. I have had it happen because the thread was catching on a notch in spool. Several other times, I though the upper thread was in the tension discs good, but it wasn't. Another time, a small scrap of thread was in the threading area and caused all kinds of mayhem. And, I do agree with everyone else, some machines don't like certain threads. Some don't like certain combinations of thread, needle and material. Batiks have caused me some grief too. Sewing, when going good is so much fun and rewarding, and not worth a darn when things are going bad. I, also, had a problem when I tried to use an "L" bobbin in a machine that only takes Class 15's. So much fun!!!! Good luck with getting yours fixed.