Crumbling paint

Hey! About 6 months ago I painted the wall that connects our kitchen to our upstairs bedroom, and the paint is crumbling off the walls... Any ideas why and what I should do to make sure it doesn't happen again.

Some context.

• The wall previously had wallpaper. We stripped this and did what we could prime. We sanded the walls / removed excess glue and we also used a primer before we used the colour paint.
• We live in an open plan house, the wall is exposed to the kitchen, with no door at the bottom or top of the stairs.
• Is this caused by excess moisture from the kitchen or is this damp / mold from the wall?

I'm planning to re do the first wall, so any advice would be amazing.

Cheers gang!