It could be the actual paint you used, either cheap low quality paint or a bad batch. Also, what kind of paint was originally on the walls? Plus, what kind of shape were the walls in, clean and sound or a little dusty?
If the walls, originally, were painted with an oil based paint and you painted with an latex, without priming, then this is the reason. Oil based painted must be primed before painting with any water based paint.
If this isn't the case, then it might be the paint you used. Inexpensive flat latex paints are notorious for problems. You might have to repaint with a very good 100% acrylic.