I put the stitch length adjuster as you told me, so now it touches the top of the raised foot. I tried with the same two layers of leather and it still won't work. Then I tried with a piece of knit fabric way much lighter in weight and thickness and the machine transported the fabric very well. Then I tried with a piece of felt, the machine stitched me one stitch transporting the fabric but then it strarted sewing on the same spot as before and only if I applied a bit of pressure pushing the fabric the foot would take it back. I don't know if it just a problem of adjusting the pressure to the material at this point. I would really like to avoid taking apart the head of the machine as I am afraid I won't be able to put it back together.
Thank you for the patience