Hollow underneath tile -- fix?
It seems that there is a bit of concrete slab cracking under my wife's tile floors over the last few years (house about 22 years old). The grout is cracked in a few places, and the adjacent tile on one or both sides of the tile sounds hollow, no longer attached. This might be for ~1/4 to 1/3 of the tile width, the part closest to the grout cracks.
If the grout is removed/replaced to fix the grout, is there a good product that can be injected under the hollow edge of the tile before re-grouting? Another technique?
The tile, of course, can no longer be found, so replacing a few tiles is not an option.