I am attempting to make velvet curtains . When sewing two lengths together , i.e. velvet faces together and on the inside , I find that the surfaces tend to move ( sideways )although pinned and also there is some stretching of one length of material in relation to the other along the direction of the sewing line . Is this a function of the machine ( e.g. tension , stitch length etc ) or is there some trick associated with the material that I am unaware of .