Hi, Kit. How did you prepare the walls before painting? Did you wash the walls with TSP or Dirtex or something like that? Normally, bathrooms are painted with a pretty glossy paint that will leave a very smooth surface. If you add a layer of oily dirt and/or cigarette smoke on top, the new paint will not stick very well to the old paint. Is this what's happening? If so, post back. Do not use a paint stripper! This will soak into the plaster or wallboard and cause you more problems.