Since the tile is flat and in the same plane the wall surface must also be a perfect match.
Tiles bulging off wall.
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.
If no adhesive is applied then the tiles will fall off.
Tiles need to adhere to a smooth undamaged surface.
Make sure that the tile adhesive that you use is water resistant.
Grouting has been done too early.
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.
I feel this in conjunction with skinned over adhesive is the root cause of your tile failure.
Any low spots on the subfloor can cause a tile to bulge.
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.
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.
Adhesive has not been applied properly.
The adhesive needs to be applied evenly and on every tile.
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.
The wall surface could have been irregular with humps and dips in it.
The subfloor must be clean even and dry before tile installation.