Did not mist coat on newly plastered wall - how can we rectify it now?

If I'm ever faced with a problem like that I seal where it is peeling with an oil based undercoat. There is also a product called Zinser non peel which does exactly what it says on the tin. Once it is sealed you then need to give the ceiling at least 1 more coat of emulsion.

Hi, It all depends on the initial coating that was applied as this is the problem. It may need a good sandpaper then clean and a grip primer coating.