Tile fell off shower wall. You should use the correct adhesives depending on the type of tiles that you are working with. The tiler suggested over the phone it was down to moisture as the shower walls were tiled on skimmed moisture resistant plasterboard. And ceramic brick shaped tiles. Water gets under it weeps up and rui. House is over 100 years old and Im tiling onto a wall with a few layers of paint on. Make sure you get a slow set adhesive and dont spread too much on the wall in one go this could have been why the tiles didnt stick to the tubbed adhesive it goes off real quick on plasterboard due to the moisture absorbancy of the pb. Mix adhesives and then apply the mixture to the area where you have just taken the broken tile off. Only apply as much thinset on the wall as you can cover with tile in five minutes.
If there was a problem with the thinset mix that weakened it you would still see thinset adhered to the back of the tile but it does seem odd to me that youve been able to scrape of the thinset without.
You should use the correct adhesives depending on the type of tiles that you are working with. House is over 100 years old and Im tiling onto a wall with a few layers of paint on. They came off quite easilly but I noticed that the wall behind the tiles has some water damage. Install new tiles to replace the broken or missing ones. Press the new tile down over the adhesives you have just applied. Wall tiles loose and fall off.