New wall tiles cracking and bowing out

[Pidders]

About 10 months ago we had our bathroom re-tiled with 15.5"x9.5" tiles over existing (smaller) tiles. We have now noticed that a number of tiles in random places on the external walls (brick) have numerous hairline cracks and are bowing outwards, although the grout between the tiles is still intact. When tapped there is a hollow sound as if the tiles are no longer fixed to the wall. There is no problem with tiles fixed to the internal walls (breeze block).The tile fitter is blaming faulty tiles whereas the tile supplier is blaming the installation. I do know that during the installation the fitter had to use thick layers of adhesive due to the walls not being true.
Any thoughts on what is to blame? We are stuck in the middle and don't want to end up paying for the re-tiling!!

[plumbbob]

This is tongue in cheek advice only, because obviously I cannot see the installation or know how it was fitted.

Tiling on top of tiles is always a risky business for several reasons. One major consideration is as the two layers of tiles are waterproof, the adhesive between cannot dry if the room is grouted too soon. This is a common mistake! It might even explain why the new tiles are cracked?

If the tiles below were uneven, they should have been removed and the wall levelled if necessary. The extra thick layer of adhesive may be outside the manufacturers specifications.

Are you sure the new tiles are hollow, or are the original ones now working loose?

If I were you, I would drag the grout out of one tile that appears damaged and remove it so I could determine exactly what is causing the bow. Be careful they don't all drop off though!