why does grout not stay?
The flooring or base below the tile is as important as the tile itself. If the floor flexes the tiles will break or pop loose the grout will crumble and come out. I like to add a cement backer board both glued and screwed to the decking. In the few cases I have used Plywood I also glue and screw it. This only takes a few minutes and it makes the base solid. With a solid base the tile will last longer. Not only by eliminating the cracking but when well laid tile over a solid base has something dropped on there is less chance the tile will break compared to a poor job. 1/4" is way thin even glued and screwed it would not hold up compared to 1/2".  5/8" would be better if using plywood. The best option in my opinion is 1/2" cement board the tile &amp; grout will stick to this better than wood. If you have the floors redone Insist on a construction adhesive or thin set be used with screws to create a solid base the few extra dollars it cost up front will stop problems like you have now from happening in the future.