There is always a remote possibility of an individual fractured tile, usually caused by not being bonded properly and then being impacted by an object dropping on it. That type of fracture is repaired by replacing the damaged tile. A sure rule of thumb to determining the problem: if the crack goes from one tile to another on a continuous line through the grout joint, the problem, without any doubt, is in the substrate; usually a crack in the slab. The damaged tile would have to be removed, a crack isolation membrane put down and the tile reinstalled; which should solve the problem.