Adhesive has not been applied properly.
Tiles bulging off wall.
The wall surface could have been irregular with humps and dips in it.
As ceramic tile get larger the wall or floor surface must be flatter and flatter.
A pull down that section of wall and re tile with new blue board etc although the current old tiles cannot again be sourced to match the exisiting.
Any low spots on the subfloor can cause a tile to bulge.
Tiles need to adhere to a smooth undamaged surface.
If no adhesive is applied then the tiles will fall off.
Damage to a subfloor is one of the most common reasons for a tile to bulge or lift from the subfloor according to the american society of home inspectors.
A buckling or partly raised tile on your shower wall usually means water has gotten behind the tile often through cracks in the grout and caused the wall material to expand pushing the tile outward.
B replace the removed tiles with a large mirror c force the wall back in with some type of ugly metal alloy strip screwed in over the top of the tiles and into the stud.
The subfloor must be clean even and dry before tile installation.
The adhesive needs to be applied evenly and on every tile.
The chalk dust will stop the tiles from adhering to the wall.
The adhesive is called supergrip ceramic tile premix adhesive as recommended by the tile shop and the grout is wateproof grout used for swimming pools again as recommended by the tile shop where the tiles were bought from the ceramic tile centre in portsmouth.
I feel this in conjunction with skinned over adhesive is the root cause of your tile failure.
Since the tile is flat and in the same plane the wall surface must also be a perfect match.