David Rodgers
Hi Charlotte, there are several explanations that can cause this to happen. The main cause is the surface you have painted over could be damp, glossed, greasy, some chemical that is reacting with the water-based emulsion or polished these could all cause blistering of the emulsion. The other possible cause is your paint is old, an easy way to know is if it has a strong unpleasant smell. As a cure you could try coating with oil-based undercoat/primer after preparing the area, scraping away blistering and lightly sanding damaged area. Hope this helps David