Becky Botello answered:
hi , I use to have that problem with my older sewing machine either your top or bottom strip is feeding slightly more. What I use to do was slightly pull the bottom strip as I sewed, but I mean slightly , gentle tug. I also heard people have different techniques some sew two at a time , like sew farbic 1 and 2 then 3 and 4 then so on after they will sew the 1&2 strip to the 3&4 strip and so on. Another is to sew is strip different directions . let me know if any of this helps.