Kim, it sounds to me like the previous owner applied a "quickly" paint job so he could sell the house. To do a quality paint job, you have to wash the walls with something like TSP that will cut through any oil, dirt or nicotine residue on the walls, patch and sand any holes, apply a primer and preferably one that will seal any stains from coming through the paint. Then you can actually paint. Residue left on the walls before the last painting could be causing your problem, accentuated by the humidity from the shower. As I said before, this may not be accurate either but it's as good an explaination as any.
You might call the painter that said that he did not want to do the work and ask him what he thought the problem was and what to do to fix it.
Just a suspicion. My guess is that you are going to have to sand off the current paint and do the appropriate prep work before you repaint. He probably knew this and didn't want to do that much sanding, which is hard messy work and hard for him to justify the hours that it will take.
It is also probable that there is lead paint under there somewhere so you don't want to sand any more than you have to. And you do not want to use a paint stripper. That will soak into the plaster.
But, again, I am just guessing. The last thing that you want to do, though, is just apply more paint before fixing the problem. Keep checking and I am sure you will find the answer.