The cause of this problem is due to applying water borne paints in temperatures below 5 degrees centigrade. But don't forget that some exterior paints are designed to chalk as a self cleaning mechanism, so always check the product data sheet before diagnosing this issue.
Apply Stabilising Solution or Alkali Resting Sealer before recoating.
Crown Trade Alkali Resisting Sealer
Macpherson Stabilising Solution
Sandtex Trade Quick Dry Stabilising Solution