QUESTIONHow do your repair hollow floor tiles and hollow wall tiles?
ANSWERANSWER - Hollow sounding tiles is not a defect per se. Depending on the substrate and installation materials and methods being used the tile can have a hollow sound without anything being wrong.
On the other hand, when some tiles or portions of tiles sound hollow and others don't that might be an indication that there are excessive voids under the tile or that the tile or some component in the tile assembly isn't bonded.
Spot bonding tiles is where 4 or 5 spots on the back of the tile are bonded to its substrate rather than the entire tile surface being bonded. This is not considered a legitimate method of installing tile unless it is with an epoxy adhesive on a wall. This means that if only 50% of the tile is bonded then the potential bond strength of the tile has been diminished by 50%. Now 50% might still achieve a substantial attachment under ideal conditions, but it doesn't meet the standards and makes it more susceptible to potential problems. It also means that those unsupported areas of the tile will have a greater propensity to damage than where it is supported, and that for exterior applications or in interior wet areas moisture can collect in those voids causing staining problems.
There is no way to repair the hollow tiles so they are not hollow any longer other than replace them. Over the years many have tried to inject epoxy through the grout joints to reach under the tile, but I have never seen it work.