Surface prep. Cleaning, Sanding, etc.
The problem is that the ceiling is not as absorbent where the PVA is, so the paint is not soaking in and is just wiping off again when you paint it, meaning it's much thinner on the PVA. I would give the whole wall/ceiling a coat of Johnstones Ultra primer/sealer (or equivalent) which will make the absorbency the same across the ceiling, then paint a coat of emulsion
North Cornwall Painting