Common as a problem with cinder block or cemented walls, they can be caused by many reasons, one of which may be a failure to neutralize the wall before painting. A freshly plastered walls should be allowed to dry, then neutralized, allowing the correct PH for the paint to adhere properly. There are however all-around paints that need no neutralizing, having within it blended, is a neutralizing agent that does it for you. Should you have this problem, the best course of action would be to strip the wall and neutralize it, leaving it to dry off for a day or two, based on the direction of the product you are using.
After the wall is dried, use the proper primer to seal it and allow a better substrate for the finishing coat to grab onto the wall. Let dry according to the instructions with the specific product, and paint away to get the beautiful wall back as it should be. Water based paints for plaster and cement-based products and oils for wood and other materials.
For external walls, it may simply be a case of the paint losing integrity, which comes after exposure to the sun and rain, that is usually listed as it’s effective lifespan. Exposed wall usually retain their paint coverings for a couple of years (5 or more for good brands of paint), that eventually succumbs to the rigors of weather and the acids contained within the rain as result of pollution. Make sure you remove all flaking paint with a stiff brish or best to power wash if you have a sizable area to clean, making sure to take into account the runoff and paint chips that would be produced by having drop cloths or some way to manage it, keeping it out of the drains. If you suspect lead paint, better get a professional to do it for you for the hazards to you, your family and the environment are far greater than the risks.