What the hollow sound actually is
A hollow note means there is a void between the back of the tile and the surface it is fixed to. That void comes from adhesive coverage. Tiles fixed with dabs of adhesive rather than a properly notched and back buttered bed will always have voids, and they will sound hollow from day one even though they are firmly stuck around the edges.
So a hollow tile is not automatically loose. Push it, and press along the grout lines. If nothing flexes, nothing clicks and the grout is intact, the bond is holding. If the tile moves even slightly, or the grout around it is cracked or crumbling, the bond is going.
Where it matters most
On a floor, hollow tiles matter more than on a wall, because a foot landing on an unsupported tile is a shock load. Hollow tiles in a doorway, at the foot of a bath, or anywhere people step off a mat are the ones that crack. Large format tiles are the most vulnerable because a void in the middle of a big tile has no support at all.
In a shower enclosure, hollow tiles combined with dark or cracked grout are a genuine warning. That combination usually means water has got behind, softened the board and released the adhesive over an area rather than a single tile.
How to test it properly
Do the whole area, not just the tile that worried you. Tap methodically across the wall or floor with a coin and mark hollow tiles with low tack tape. A single hollow tile in a sea of solid ones is a fixing defect on that tile. A cluster, or a run following the joists, is a substrate problem.
Also check the floor for deflection. Stand on the area and rock your weight. Timber floors that flex under load will keep breaking the bond no matter how well the tiles are refixed, which is why proper preparation, ply overlay or a decoupling membrane, matters more than the adhesive brand.
What the repair involves
An isolated tile can often be lifted and refixed. The grout is cut out with a grout rake or multi tool, the tile is broken out carefully from the centre so neighbouring tiles are not disturbed, the old adhesive is chipped back to a flat surface, and a new tile is bedded with full coverage and grouted to match. Matching grout colour on an older bathroom is the hard part, not the tiling.
Where a cluster has failed, or the floor deflects, the honest answer is that patching is temporary. The area needs lifting, the substrate stabilising or replacing, and re-tiling. We will tell you which of those two you are looking at before quoting.
Getting it checked
It is worth a look if hollow tiles are spreading, if grout keeps failing around the same ones, if a tile has cracked with no impact, or if the hollow area is in a shower. Those are the cases where the sound is telling you about water or movement rather than adhesive coverage.
Send a photo and a rough idea of how many tiles sound hollow through the enquiry form and we will tell you whether it needs anything doing at all. Plenty of the time, the answer is that it does not.