Soft crumbly finish to multi finish..any advice?

[Mark Huddleston]

Hi all. We decided to remove the decorative coving in our living room and go with no coving for a modern look. I’m your average DIYer and have plastered small areas before using multi-finish. Coving and adhesive was stripped off. Internal corners were taped and filled with easifill. PVA mix over the area and then 2 coats of multi-finish feathering edges out. One wall/ceiling has dried fine but on one stretch of ceiling the plaster has dried soft and crumbly. When I’ve gone to use a fine sandpaper it’s just come off way too easy. I think I used the same technique of applying the second coat before the first coat dried. Could this be inadequate mix of PVA? Dried too quickly? Contaminated water? I did trowel it quite a bit and my wife kept saying “leave it alone”. I would hate her to be right but maybe she is??
I’ve sanded it back and may have got away with it but I’ve got 2 more wall/ceiling corners to do so wondered if anyone could suggest what might have gone wrong please?
The photo is the wall that has dried OK. It does need a sand into the corners as it’s my first time so be gentle please!