Damp showing up in new plaster

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I’m renovating a relatively new (70s) building in Portugal. We have plastered the walls (with a trustworthy tradesman) and over the last 2 months damp patches have started to appear on the party wall shown, which seem to be getting worse since we’ve had cold and wet weather over winter (pic with jacket is this January, other is November last year) There were also signs of a little damp/flaking paint before we plastered but nothing drastic and it seemed to have been dried out.

It’s a concrete wall, doubt there’s much in the way of insulation or waterproofing but there’s no damp in the floor above so it doesn’t seem to be a problem with the roof. Worth noting that the house isn’t heated and we don’t have a dehumidifier going, but this seems to be the only wall with this problem.

Wondering what to do here, just try to dry it out or build a new plasterboard layer with a cavity/membrane?! Any advice would be greatly appreciated.
 

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Might it be the case that they are historical stains that have simply bled through the new plaster?
 
Might it be the case that they are historical stains that have simply bled through the new plaster?
Any way to fix it other than covering with more plaster board? We’re going for an exposed plaster look on this project so can’t just paint over unfortunately
 
No staining above does not exclude roof problems , damp often travels to ground level before exiting to the surface.
 
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