Sounds very much like they have been laid on an unstable (wooden?) floor. If the floor moves in any way the tiles will either crack or move with the floor which means the grout in the joints will crack and form holes. There is no way to overcome this problem except to strengthen the floor. It could also be that the floor has been laid on a bed of adhesive which is too thin and consequently is not supporting every bit of tile. If the tile wobbles the same symptoms will occur.