Try checking the pressure regulator on the top of the machine above the pressure foot.
http://www.westsidesewing.com/wss/tips/art_pt08.pdf
Info on pressure regulator
http://www.ask.com/questions-about/S...e-Presser-Foot maybe something on there on how to use the pressure regulator
http://www.ehow.com/how_5976861_chan...g-machine.html