Painted straight onto plaster

Plaster is very absorbent and needs a watered down paint or primer straight onto the bare plaster otherwise the plaster absorbs the water from the paint and causes it to crack and peel, the only way i know to solve this is to scrape of the paint and re-do the painting but dilute the emulsion with 50% water, i have had to do this a few times. If its not flaking everywhere you could just scrape of the lose bits then sand down the fat edge or fill and smooth it. Eventually it could all start flaking but its a temporary fix. Good luck.

You should never apply a vinyl matt emulsion directly onto a newly plastered surface, the flaking is caused by the vinyl content of the paint reacting with the plaster, the more polished the plaster is the more flaking appears and it will only get worse, the best way to avoid this is by using an actual new plaster primer, can be costly or a less costly method of using an obliterating emulsion, eg Johnstones jonmatt, or an eclipse emulsion, these products have no vinyl content and adhere very well to the plaster without flaking. Stevie SMD Painting & Decorating

You can buy new plaster paint from most builders/paint merchants. It is specially made for new plaster.