Had the same problem in my sons bedroom and cured it by spraying with a good bleach, domestos, and leaving a while and then washing off with damp cloth. Leave to dry and then paint over with polycell damp stain . It is brilliant stuff. Smells for a bit but the damp has never come back.Finishes white but let it dry and then emulsion over.Quite expensive , about a tenner for a small pot , but you get what you pay for, B and q sell it.
My sons damp was also in his bedroom as he would never open his window. No matter how cold try to open a window at the front and back of the house once a day for about an hour to get a through draught to ventilate the room