Was the slab fairly new when you put the tiles down? I am assuming that a crack isolation membrane was not used. It is always recommended when tiling over concrete slabs as slab cracks can happen at any time. You mentioned that the tiles are cracked in several different areas. It is possible to have several slab cracks develop. Normally, though, with slab cracks transferring to tile, you can see the progression of the crack across the tiles. Sometimes, however, some tiles crack while others don't. That is because the bond has been broken between the tile and the slab before the tile had a chance to crack. You can tell by knocking on the tiles. If it sounds hollow, the bond has been broken, but your tile has been spared of a crack.
Michael Prachniak - Expert Tile and Stone Consultant and Writer, Owner - TileProductions.com
Check out FAF''''s tile section!