Hi, don't know if anyone can help but I'm having real difficulty getting tiles to stick to the current wall in bathroom. I was hoping on removing the old tiles that the surface would be ok but not great to just tile over. However every tile is not grabbing well and falls off from the slightest pull. The tiles are 300x600 ceramics. The mixture used was mapei standard set adhesive ( powder). Tried a few different consistencies of adhesive and nothing seems to hold. The walls are in pretty bad way. Pictures attached. I need help with knowing if i need to get it plastered, back borded or primed first. Dont want to plaster if it will make no difference. Any advice would be helpful. Thanks.