most likely wrong grout and wrong adhesive.

I recomend Flexable adhesive for all bathroom locations. Is it a solid floor? or Timber Floor? If it is upstairs then I would guess a timber floor, with floor boards? If this is the case was Ply or Chipboard layed first, over the floor boards, prior to laying tiles? This is a MUST - ideally at least 9 mm thick should be used.

Without knowing the precise "structure" of your floor it is difficult to accurately comment, but hopefully this will help.