I recently acquired a used Dressmaker machine with the same problem and found that when I changed the needle that it had had a needle with a short shank and therefore would not go down far enough to take the thread around the bobbin case. The needle change corrected my problem but your timing may also be off. Check www.sewusa.com for detailed instructions on correcting this.
Ann Brown
September 2007
make sure the bobbin case is in properly! Re thread top thread, hold on to thread as you lower the needle and slowly bring it up again. If this doesn't work...change the needle and repeat.MAKE SURE YOU'VE GOT THREAD ON THE BOBBIN!!!!!!!!
Sounds stupid but you wouldn't be the first!
Ann Wilde
October 2006
I can answer this question...
Why won't the needle thread pick up the bobbin thread?