Floor tiles cracked and not stuck to floor

When the floor was initially laid the adhesive underneath was not spread adequately, leaving areas with no coverage causing weak spots. Over periods of time depending on the amount of traffic the tiles begin to become loose and sometimes crack.

The adhesive hasn't been spread out equally to give fully coverage of the tiles causing them to crack in void spots and become unstable to the floor

It can be down to a number of reasons: Was the floor primed prior to Tiling, was the floor grouted before the tile adhesive had set and as above, if the adhesive hadn’t been spread evenly with the correct sized notched trowel, it will create a void in the adhesive. Personally, I always back butter the tiles prior to laying them down on the adhesive, that way, you get 100% surface contact.