When you replace the tile make sure the adhesive is applied to the wall not the tile. It is best to use a serrated scraper to apply the adhesive to the centre of the area, using the minimum of adhesive, (for a 6" tile a maximum of 4 square inches of adhesive should be used in the centre of the area,) this allows the new tile to seat properly and level with the existing tiles. A big blob of adhesive is impossible to squish down behind the tile.
