Platered ceiling - pot marked surface.

I have just had a a bedroom ceiling and walls re-plastered. Before painting the surface looked very good. So I got on with the job of painting.
I Got some Wickes Trade white emulsion and watered this down by about 10% for the first coat. After the first coat the ceiling and one wall looked a bit blotchy but the painted surface looked smooth. Next night I gave the ceiling a second coat however when dried the blotchy areas came up pot marked. Texture like some poor soul with bad acne. It is as if the paint had not stuck to part of the plastered surface. I tried to even this out by painting the worst areas again but this seemed to make it worse. So following night I went over the worst areas with a sander making the surface without going back to the plaster. I then repainted last night and again the areas look pot marked!!!
Could the problem be down to the first coat. IE not having mixed the water into the paint properly. If so will sanding back to plaster correct the problem?
Could it be a problem with the plaster. could it be contaminated with something. Should I get the ceiling re-skimmed.
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