i'm thinking the same as trini. start from the centre and work out. but i also think this is like when you're quilting and need a walking foot because the top layer is moving along at a different speed than the bottom. the bottom moves faster and the top gets pushed down so that it finishes with it's lower end even lower than the backing.
maybe the batting is too thick? maybe the fabrics are too heavy for the frame set-up? maybe you can start in the centre even in the frame and work outward toward each end while on the frame?
have you used the frame before? have any trouble with it? i had this problem with my last frame when i tried to get too much on the roller at one time. it creased between rows of horizontal stitches. i think it was wound too tight to fit it in too small a space and got all squooshed.