POSSIBLE CAUSES:
Failure to maintain a "wet edge" when applying paint.
Applying 1 coat of masonry paint where a 2 coat system is specified. Where overlap occurs, these areas in effect receive 2 coats and therefore uneven durability results.
SOLUTION:
Maintain a wet edge when painting by applying paint toward the unpainted area and then back into the just painted surface. This technique (brushing from "wet to dry" rather than vice versa) will produce a smooth uniform appearance.
It is also wise to minimise the area being painted, and plan for interruptions at a natural break, such as a window, door or corner (especially important when applying stain to bare wood).
Ensure the specified number of coats is applied.