Hey there. I totally feel your pain. All of my early box cushions and panels did this too! I learned it’s because the fabric layers move under the sewing machine at a different rate, causing it to slip and move as you sew. This is most cumbersome with domestic machines and is alleviated a little bit when you use an industrial because they hold the fabric way more tight.

A great way to improve your approach is to sew your cushions from the middle out, starting in the back, then moving to the front. My favorite online instructor Cynthia Bleskachek made a great instructional video that is now owned by the Workroom Channel.

Here’s a link: https://www.youtube.com/watch?v=5drVgRcGd6c

My cushions have been perfect ever since I began implementing this method! I hope that helps!