Hi queen, what was on the walls before you painted? Would help to know if there had been paper up, or if you have recently moved in and it's your first time of painting these walls. You say you have painted many times over the years but don't say if it was at this property. There can be a number of reasons for this happening, pva on walls, paste still on them, sealer coat not applied when done first time. A bit more information would help.