Alternate your seams. Sew the first one top to bottom, then then next one bottom to top. Press after each seam, don't wait until the end. If you have an ironing surface with a grid, this will help you press. In addition to a strip being cut with a slight bow, or a wavering seam, the tension on the strips may be uneven causing it to not lay straight. In other words, either the strip on top or on the bottom is pulling thru the machine at a slower rate than the other one. Try using a walking foot or lightening up your presser foot pressure just a bit & make sure you are not holding on to one of the strips as you feed it thru your machine.