why has a mould patch started in bedroom?

I scraped off the paper washed it down with sugar soap, repainted with bathroom & kitchen paint, looked good for two months, then mould started growing in corner on outside wall at ceiling corner, how can I cure it? would puttin vent in corner work?
reuben, January 2012


done
Dampness or gutter maybe full or leaking.. wash the wall and repaint using an anti mould paint cost about 27 pound for small can

Martin Mcandrew painter from thirsk north yorkshire, February 2012
Not enough ventilation will cause the problem you have as the warm air condenses onto a cold surface, ie, the outside wall. Open the windows more often, or leave the window partially open when you go to bed, simples.

Alexandre Meerkat, January 2012