Hair line crack in old plaster/paint

This room was plastered 10 years ago, sound insulation board put up and taped, plastered over (didn't work, can still hear everything 🤪)

After painting and decorating, months later, we noticed cracks form months later, some of which we think are where the boards meet.

6 years ago we wanted to paint so raked out the cracks and filled, sanded and painted. Looked great for a few months then the lines formed again. (you can see my filling and sanding up close in the photo)

Is raking and filling the wrong solution or will these always show up again regardless because of general movement?