We recently laid a ceramic tile floor in our kitchen. We installed 1/4" cement board and then the tile. When we tried to grout, the following morning we noticed around many of the tiles, the grout had crumbled and actually come out! We dug out all of the grout and bought new grout, and tried again; same problem the next day! The floor is level in almost all areas except for one, and that grout is fine! Is there a flexible type of grout that would withstand some minimal movement of the tile or do we need to jack up our joists from below? We just want to finish this ongoing headache