Ken,
Great answers from Vicky. I've had this problem when the fabric has been too thick or the there have been too many layers. If that isn't the case, then sometimes I have had to start sewing in from the edge. I also start the seam with the needle in first then lower the pressure foot. Next I have had problems when the threading isn't right - if the fabric is stationary you can quickly get a build up of thread underneath then nothing is moving. I have one machine that is a bit 'slow' that way so I start with a little pressure from the back pulling the threads/fabric. Hope this helps.
ddjx2001@yahoo.com
Canada
D. Jorgensen, August 2007