Kitchen floor with underfloor heating seems 'lightweight' and hollow in places

This is because the floor tiles and underfloor heating has been installed incorrectly when tiles sound hollow there is no adhesive under the hollow spots , the tiles are cracking because of this . when installing tiles you need a bed of adhesive evenly spread to fix the tiles in place,also your underfloor heating need's a thin layer of latex self levelling compound to cover the wire matt protecting the wires and giving a level surface to lay tiles onto.You will have to take the tiles up and if there's no self levelling screed above your floor heating system it is likely to be damaged when taking up the tiles. Kind regards , Stephen

Improper installation