Some of the older sewing machines have a presser foot PRESSURE adjustment - on many machines it's the knob on top of the machine above the needle bar - I found that on my Singer 237 (a 1960s "economy" model - tan) that I could sew on a lot of thicknesses when I reduced the pressure. Before I reduced the pressure, the top layers would move forward as I tried to sew.
Anyway - it's another adjustment to consider.
It also helped to lengthen the stitch.
And to use a heavier needle - like a 90/14 or 100/16 when I was using denim.
I have used old thick terry towels for the middle of potholders.
The suggestion to test the combination under a hot iron is an excellent one.
Last edited by bearisgray; 07-08-2014 at 04:08 AM.