Couple of tiles fallen off wall after tiling?

Done tiling many times before, and never ever had this problem. Two of the top row of tiles have fallen off (I'm told on their own). I tiled straight onto painted wall with standard ready mixed wall tile adhesive. Evidence of a lot of moisture behind tile when removed, it was completely dry before. Is it a matter of damp in wall and after two days will just take longer to set? Or is it a odd paint which the tile won't adhere to? Help much appreciated. I would hope when the grout is on that will help stick them together until they fully set?