Very common. I find the mildewcides to be virtually worthless. Mildew is just gonna grow on siding that is damp most of the time. The spores feed off the paint. I have even used a high gloss paint on siding thinking the mildew could not stick to a shiny surface. Had more mildew on that surface than any other. Wish there was a magic solution, but dampness that clings to siding for large chunks of the day is just an incubator for mold growth no matter what you do to fight it.