There is a tiny hairline crack in between two tiles on the grout on the side where the shower unit is installed. It is at the bottom of the shower near the shower tray. It was fixed once but it has come back. If I push the tile, it appears to be moving a little. There is plywood behind it. Can I get away without doing anything? If not, do I have to take the grout out and reapply fresh grout. How do I stop the movement of the tile? Thanks in advance.