The other day I was doing FMQ on a table runner. I was using 40 weight thread. I had two different threads for contrasting work-from two different manufacturers. I was using a 14 topstitch needle. I was using 50 weight, 3 ply cotton in the bobbin to match the backing. One thread stitched in beautifully but the second one had top stitches showing on the back of the runner. Don't understand that-both threads were 3 ply 40 weight. I did not reset anything and did not turn the machine off when rethreading from one to the other. I switched back and forth two or three times just to see if there was a 'me ' factor. The results continued to be the same. I hope laundering the finished piece will help.