There are numerous possible causes for this. Applying the paint over unstable, older, coatings; failing to prime unstable or powdery surfaces; or failing to either prime or sufficiently dilute the product on bare or highly porous surfaces. Moisture can also cause flaking
Remove all loose, flaking, or suspect, coatings. Prime or stabilise unstable surfaces. Dilute coatings as instructed on product data sheets or on the back of pack for bare or highly porous surfaces.
Macpherson Stabilising Solution