Wish I could help you out on this one, Harry. I can't say that I've experienced the same thing. I've had things that took a while to dry, but, they always dried in time. Plaster is one of those surfaces that seems to take a while to dry. I guess if it has some porosity to it, it's going to dry much faster as the primer and paint tend to get soaked up into the substrate. Plaster is hard and nonporous so it takes longer to dry. Anyway, I'm rambling, but, try having a fan running or a dehumidifier to speed up the process............