i personaly wouldnt wash the ceiling with suger soap unless it was a kitchen ceiling that would be greasy, get a pole sander and sand it down with a fine paper thats if it a smooth ceiling. if it a smooth flat ceiling and its quite a big area try not to roll the ceiling length ways, do it width ways so you have a shorter area to get back to the wet egde. i also found in the years ive been painting that matt finish paints are a real pain to get a good even finish with no patches, specialy in this hot weather because it dries way to quick an makes the finish un even. try kool the room down if it is warm, also could be air movement witch courses it to dry un even so close the windows,so it could be either one of those.