What tension are you using? What is your stitch length? It sounds like you are quilting and not piecing. Are you absolutely sure that your bobbin is threaded properly? I had a similar problem and just as I was ready to pack it into the dealer i found I had missed a little place in the bobbin area that the thread had to hook over. Another time I had the bobbin in upside down. I have the Janome MC6600P so can't speak for the Artistic SD18.