You may not notice that a tile is coming loose until it shifts underfoot or makes a noise when you step on it. Loose tiles can be caused by moisture, cracked grout, an uneven subfloor, or insufficient mortar. Methods of fixing loose tiles abound, from injecting epoxy underneath the tile to removing whole sections of subfloor to find the underlying problem. If just one tile is loose, you generally can repair and re-grout a small area.
Set the blade of a putty knife on the grout lines surrounding the loose tile. Chip out the grout, lightly tapping the end of the handle with a hammer if necessary. Brush old grout chips away with a whisk broom.
Work the edge of the putty knife under the edge of the loose tile and lift up the tile. Turn the tile over and gently scrape away loose mortar from the back. Set the tile aside.
Scrape loose mortar off the subfloor with the putty knife, angling the blade and tapping with the hammer to loosen the stubborn bits. Whisk out loosened crumbs and vacuum.
Mix a small batch of new mortar, following package instructions for measurements. Apply the mortar to the subfloor with a trowel, leaving a smooth, even layer. Turn the trowel over to the notched side and make a pass over the mortar to leave ridges.
Drop the tile into place and press down to seat it in the mortar. A little extra mortar may come up through the joints. Wipe it away with a wet cloth. Allow the mortar to dry for two hours.
Mix a small batch of grout, following the package instructions. Apply the grout to the joints around the tile with a putty knife. Draw a damp sponge across the new grout lines at an angle to remove excess grout. Allow the grout to cure for at least one week before washing the floor.
Buff away any residual "grout haze" with a soft cloth a few hours after applying grout.
Avoid stepping on the repaired area for a few days until the mortar and grout are well-set.