When a tile debonds from a surface, it’s important to take note of where the failure occurred to help determine the cause. Did it fail adhesively between the back of the tile and the thin-set mortar or adhesively between the substrate and thin-set mortar, or did it cohesively fail within the thin-set mortar or tile? If the thin-set mortar only bonded to one surface, either the tile back or the substrate, then the problem is likely installer error and not the result of a bad batch of thin-set. Obviously thin-set mortar adhesives are made to bond to both surfaces. So if the thin-set bonded to the tile but not the substrate, it would suggest that the substrate had some sort of contaminate or condition that interfered with the bonding of the thin-set. One of the most important steps in tile or stone installation is preparing the substrates. Clean them and remove all surface residuals. Make sure water readily absorbs into cementitious substrates, and if it doesn’t, scarify the substrate to open up the pores. It’s just as important to clean the backs of tiles before installation in order to remove any residue that could act as a bond breaker, particularly on stone tiles.