If I had something that persistent, I would cut it out and replace with new plasterboard and coving. It sounds like there is some sort of
structural problem in the coving & plasterboard (assuming it's plaster coving/plasterboard).
I've got this on a small plaster edge in my kitchen. Paint will just not stick to it, something in the plaster. It's not a large area, and it's not constantly affected by damp. Must get round to it one day.