Floor tiles hollow and cracked

Its possibly down to lack of adhesive, or the substrate applied was not done correctly.

I would presume that who ever layed the tile may have layed them on spots to reduce the amount of adhesive they was using not good practice also see this on patios.

They have not had full coverage of adhesive. Wall tiles can be fitted with a 60% adhesive coverage but floors must be 100%

Potentially a number of reasons but if they sound hollow then it's most likely someone had "dot and dabbed" them in. Coming loose is more likely the prep work not done correctly. I'd take a guess and say the surface may not have been primed properly or it wasn't a suitable substrate. Hope this helps.

Without viewing I would look at three possible outcomes 1: The tiles have not been fully bedded into the adhesive and the tiles have been laid on circle balls of adhesive(dot and dab) 2: You have porcelain tiles/underfloor heating and the tiler has not used a flexible adhesive suitable for these, or alternatively underfloor heating was turned on to quick. 3: The floor surface has not been prepare correctly for tiling onto, lack of primer of laitance has not been removed from anhydrite screed.