Plaster problem after chimney removal

[karlosmaximos]

A few years ago I took out the chimney breast to make ground and first floor rooms bigger.
The house is a mid terrace with a gennel/alley running up between properties where the chimney was.
Problem is:
On the ground floor about 7ft from ground there is a patch of plaster (about 9" diamater)which refues to take paint, and flakes off over time. I unibonded the wall,bonding,top coat etc when I did it.
I cant see it as damp as it doesnt ever spread or show anywhere else.
Has anyone any suggestions as to the problem?i'm wondering if the unibond or bonding has had a reaction with some soot that might have been left on the brickwork?Its as if the plaster has never dried out.
I'm confused.com

 

[Nige F]

does it look like a Saint

 

[stuart45]

google hygroscopic salts.

 

[joinerjohn]

Sounds likely that there was a patch of soot on the internal brickwork of the chimney. Does this patch look like it's always damp? (but isn't) I've seen this before on walls where chimney breast has been removed then plastered over.

 

[karlosmaximos]

Ill Google that definatly,yes it does look like its damp but isn't

 

[karlosmaximos]

Ill Google that definatly,yes it does look like its damp but isn't