A walking foot is great to use with that much bias stitching. Some machines have an even feet foot.
It could be the pressing though if you are "ironing" on the seam. Pressing means pressing down straight lifting and pressing down straight. Don't scrub! A clapper set onto the seam might help too. Please use a dry hot iron.
I have seen the ironing board covers with straight lines printed on them to assist you with keeping things squared up as you go.
Last edited by RedGarnet222; 01-12-2019 at 09:30 AM.