About two months ago I had my downstairs toilet walls tiled (solid walls, one interior, one exterior, painted and sealed) with ceramic tiles from wickes using wickes flex adhesive/grout. Last week I noticed some marks on the tiles and realised that they had started cracking and bowing away from the wall, although the grouting has remained in place. They are still attached to the wall and it doesn't sound as though the plaster has blown - no hollowness when knocked. I've read other comments on the forum that sound similar but not quite the same. Wickes have requested that I remove one of the tiles and take it in to them so that they can have a look but I am loath to remove any tiles until I have some idea of the problem. Any ideas as to what might be behind my problem - pics attached.