Chalky paint

[Benfield]

Hi all, I prepared the spare room for paint to use Crown Pure Brilliant white Matt. The walls were stripped of old wall paper, washed, dried and then a coat of Zinsser was applied. The under colour was purple so I allowed for 3 coats of white, however a year later the walls are chalky when you rub your fingers over them. Is there anything I can do as am unsure what the problem is, the window (small window) is permanently open all year round?