My house was plastered a year ago but the plaster doesn’t look dry

Without looking I would 100 percent say the plaster is dry from the initial work done over a year ago …..it would seem you have some water getting in and the plaster is soaking up the water causing the damp and the paint to peel ….I would recommend you get a damp survey doing , we have had a lot of rain and cold weather which may of added to the issue thanks

Agreed ,no way the initial plastering would still be wet .Dry plaster will act like a sponge and become loose and weak if there is an underlying damp problem which it appears there is.Aquire the services of a damp surveyor and perhaps a multi trader to locate your problem.

If the wall backs out to an external wall check for cracks missing cement or even leaking guttering or down pipe that can cause a lot of problem

I think you might need to check the pointing on the brickwork or cracks in the render when water gets into the cavity it keeps the insulation wet and it won't dry you might need to have your cavity cleaned out

I always go to the cheap option ist there’s many places that could be letting in the damp drain pipes roof tile with small crack it runs down the cavity and could catch on a wall tye and creep in this way look to see if there’s areas letting in water etc a mastic gun joint can fix the problem it could be were the previous platerer didn’t apply the backing correctly so it’s come away from the wall basically dont throw money away on a damp investigation until you have done some homework yourself as it’s money away and probably isn’t cheap for damp experts I would get a platerer to give you a quote that way someone in the know comes there with a free estimate