Roger - if you used oil stain that is why. The alkyd binder in oil stain is actually food for mildew!
Spray your house with a bleach water solution - rinse with a hose - then pressure clean.
Let dry for about 30 days - then prime any bare spots with a good exterior oil primer. If you have any more than about 40% bare area - prime the whole thing.
Two coats of BM XT Ltx - flat or satin finish - your choice of colour would be fine. I would recommend Glidden Evermore from The Home Depot - or ICI Dulux Ultra (not Dulux Pro) Exterior from ICI Paint stores as well.
My 163 year old New England house has Glidden Ultra-Hide Exterior Oil Primer - and two coats of ICI Dulux Ultra Flat......and it looks great!!