grout and underfloor heating

Just finished tiling the bathroom floor. I have electric underfloor heating and despite not switching the heating on for two weeks after the floor was grouted I have found that the grout has dried up and is flaking away. Had the tiler return and regrout but again the problem has reoccurred - Any suggestions what I can do?

FIrstly, the heating shouldn't be turned on for 28 days, it take the grout that long to completely cure.

What grout did he use?

All the materials need to have flexible admix in to cope with the expansion and contraction of the heating system.