any ideas

[markos1]

hi guys just a quick question, i went to price up a job today it was the wall on the stairs that goes from top right through to the bottom its just a simple reskim however one thing perplexd me, the house was built 25 years ago and all the plaster work inside is the original, when i looked at the wall i was priceing you could see all the motar joints of the inner skin of blocks through the plaster and paint they were just showing up as dark but quite wide lines all over the wall and i couldnt think y i mean a few ideas floated about but nothing that really suggested to me the reason.
does anyone have any ideas
thanks alot guys
markos1

 

[joe-90]

It's got to be a damp issue somewhere along the line.

 

[markos1]

i had originally thought this but its consistant over the entire wall and there is no areas of black staining other than the mortar lines of the inner skin block work and no noticable tide marks,and it has a decent size cavity on the place. also if it is that then that would mean the entire wall is damp and is a big wall and doesnt have that tell tale cold to the touch damp feeling.
yours confused
markos1