Why is the paint coming off the wall?

Do not under any circumstances seal plaster with PVA, this could be the reason paint is peeling in the first place as emulsion is designed for painting over porous surfaces. First of all I would try to rule out damp as an issue so seeing as you have stated it is an inside wall i would investigate any pipes that may be above the area usually when there is a leak (depending on how old the leak is) you will see some yellow staining on the ceiling if this is the case first rectify this and wait a few weeks for the damp to dry out then proceed to remove all loose paint bearing in mind if the loose paint is on quite a large scale you may want to re skim the wall as it may be too much to fill in order to achieve a good level of finish. If no signs of damp are present I would suspect that poor preparation of the original paint work is to blame i.e sealed with pva or not mist coated with a diluted emulsion in this case you should be able to remove most if not all the loose paint, any hard stuck areas can be sanded down, prime the wall with a white emulsion thinned with about 30% water and paint with a further 2 coats in a colour of your choice. ( allowing to dry fully between coats).

It could be that who ever first painted the fresh plaster did not seal it with a mist coat of paint.scrape back loose areas when dry and paint with brush or sprayer not roller allow to fully dry between coats.

it sounds like you have some damp in the wall,so the paint is not ahering to the wall you need to strip the wall right back dry it out if possible and then coat it with a damp proofer, and then seal it with a 50/50 mix of pva and water, i would give it two coats and leave to dry for 24 hours, hope this helps, Dave