How to get rid of dark patches on painted plaster?

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Hi all,

I've recently repainted the spare room and when the paint dried I can now see about 6 dark circular side plate sized patches.
Now i've applied another 2 coats but I can still see these plate sized patches.
It's not damp. Any idea what's going on? Do I just need to put even more coats on?
Dunno if it's relevant but walls were originally white and now we've gone for a light green
The whole house had new plasterboard and was reskimmed about 7 years ago.

Cheers!

 

[MdB]

If it is a non breathable wall then just shove on some stainblock and then repaint. This covers most things though I did have problems with a yellow stain coming through stainblock once (three coats!) which interestingly a simple coat of trade emulsion covered up fine.

 

[spot]

Just in case anyone was interested....
It appears the plasterboard was more absorbent in some areas than others. These patches were the more absorbent areas a just took longer to dry out.
I made it worse by putting more paint on theses patches thinking I would "cover it over".
D'oh!