Paint bubbling/flaking along edge of new plaster - is this the right approach?

[aargh]

Background: Because of some water damage (long ago - it's fully dry now), we recently had part of a wall replastered - due to access issues etc. it was easier to get just the damaged part done rather than the whole wall. But of course this did leave a boundary between the freshly-plastered section and the rest of the wall. (There was also lining paper on it too I believe, which maybe complicated things a bit more).

Anyway, I then sanded a little where there were some slightly lumpy edges to the plaster, did 2 mist coats and then 1 coat of matt emulsion - all quite well spaced out and some time after the plastering, so I don't think not drying sufficiently was the issue.

Mostly it looks great now, however the exception is along the "tide mark" where the fresh plaster ended - along this line, I noticed the emulsion was starting to bubble up while painting, and once dry, it has cracked and flaked.

I'm guessing the solution is to sand along the flaky section, maybe sugar soap again, apply a bit more watered-down paint if this exposes the plaster again, and then retry the emulsion - but please can you confirm, or is there anything else I should be doing as well or instead to try and fix it? Most of the wall looks tons better now so it would be a shame to not be able to get that part looking good too! Thank you!

 

[blup]

Sounds like the plaster didn't fully dry out or there is another source of damp.

Blup

 

[aargh]

Thanks blup. I don't think it's damp though because it was quite a while between the plastering and first painting, and also, the bit which had the damp before (and in fact all the rest of the wall) is totally fine now and no flaking. It's literally only the boundary between the new plaster and where it meets the original wall that's doing it, so it seems to be something about that edge. Maybe it wasn't smooth enough or something? Though the plasterer had done his best to smooth the join.