If the material is pulling unevenly, it is caused by the feed dogs. I've seen where the gearing to the feed dogs gets worn and quits pulling the material altogether. If you are consistently sewing heavy denim clothing you may want to look at an industrial machine.

And like Kay said New needles do wonders even when you don't think you need one change it they are the least expensive of sewing machine repairs.

Get out the instruction manual and find the maintenance information. Take the covers off and clean and oil all of the locations shown on the drawing. While apart, remove any lint, dust, small pieces of thread, etc. You need to do the left cover where the needle is, the top where the gears are, and the underside where everything is attached. Don't forget the bobbin holder compartment.

Have you tried a new Jeans/Denim needle? Is the machine clean? Are the feed dogs clean? It's amazing what a difference a sharp, new needle can make. See: http://a1sewingmachine.com/dull-needles....

I have a singer 7703 and it just started to not go through several layers of denim and it has also been pulling the material unevenly. Is there any reason for this? I think the motor on it might be starting to wear out.