Hi, if you try doing what I said about sanding down and giving a couple of mist coats first that may do it. If when doing it the first time the paint was put on straight from the tin then it may not have adhered properly. A mist coat seals the wall and lets the paint stick to it. If the paint was put on straight from the tin then sometimes it sort of floats on top of the plaster and occasionally it will bubble or peel. You wouldn't always notice this at the time of decorating. But don't use PVA at all as this is known to cause this type of problem and something similar will have beeen in the tile adhesive so make sure it is all gone before repainting. I have had this happen to me when I thought I had got all the tile adhesive off and the bubbling only appeared where this had been and I worked in the trade for many years.