Why do my floor tiles sound hollow when I walk on them?

Mine were hollow from day one on a new floor — a couple eventually cracked. Had just those few lifted and re-bedded rather than the whole floor.

For anyone planning new tiling and worried about this: hollow/drumming tiles come from voids under the tile (poor adhesive coverage or an uneven substrate), and it's an even bigger risk with big tiles — there's a newer thread on why large-format tiles need a flat substrate and full back-buttered adhesive precisely to avoid the hollows and lippage. Getting the coverage right first time is what prevents the drumming you're hearing.

Worth adding for anyone whose tiles have gone hollow: a big hidden cause is the wrong or too-basic adhesive under the tile, not just poor coverage. Standard cement barely grips dense porcelain or large-format tiles, so they debond and sound hollow even when laid 'neatly'. There's a newer thread breaking down tile adhesive types — cement vs flexible vs epoxy and which to insist on for porcelain, wet areas and big tiles. If you're relaying a hollow floor, get the adhesive spec (and full coverage) right this time — it's the bit you can't see once it's down.