Heya! It’s hard to tell from the image, but based on your description, it sounds like your back panel isn’t long enough, because when it reclines, it stretches out, and the velcro becomes unstuck. Try adjusting the length and cut a longer piece. Determine how much you will need by measuring the recliner while reclined.

This is why I encourage my students to NEVER use the existing patterns of their projects if you can avoid it. When you reupholster a piece of furniture, it is taking on a new shape with new, fluffier materials, and the older patterns, all stretched out and worn from use, will not fit the same way.

That may or not be the case with your project, but it was a lesson I learned early on. LEt me know if that helps!