Moisture causes blisters, where the paint lifts from the surface in little bubbles. It usually occurs when moisture seeps through exterior walls into the home, or through damp interior spaces like bathrooms, to the exterior surface. It can also happen if paint is applied on a damp surface. Check for sources of interior and exterior moisture, and deal with them by installing vents, exhaust fans or sidings as appropriate. Then, scrape away all loose paint and sand the surface. Repair old caulk. Once the surface is completely free of old coatings and fully dry, you may repaint with a high quality water-based paint.