I think the problem may be something called "Frosting" since this is a dark paint you used. It's caused by the calcium carbonate extender pigments dissolving in any rain that falls on the paint, and then drying on the painted surface to form a salt that's white and powdery, very much like chaulking.
The recommended solution is to remove the calcium carbonate by sanding it off, painting over the affected area with an oil based primer (that will prevent the moisture from penetrating into the paint) and repainting.
Click on the grey "Solve your paint problems" link
Click on the peach "Exterior" link on the right side
Look at all the different paint problems, especially chaulking and frosting.
Does either one look like what you're experiencing?