The reason you are having problems with tempera on plaster is that tempera is NOT the medium to use on that material. Plaster forms a skin when it dries, and it does not allow paint to penetrate very well. The plaster skin has a tendency to slough by itself, and the tempera sloughs with it. I stopped using tempera many years ago, around the time acrylics came into fashion. Tempera is not flexible, and has been known to flake off paper, so I would not expect it to be any better on plaster. Acrylic is almost as inexpensive as tempera, so cost should not factor in your choice of paints. It has the added benefit of being very flexible, because it is essentially a thin coating of plastic, which will seal the surface of the plaster as well as give a durable, moisture proof painted surface.