The grout has voids [Photo 1]. Usually the voids are represented by a thin film of grout covering the void. Over time this thin film either breaks or deteriorates exposing the void beneath it.
Technical reason for voids in the grout
Morgan advises that the grout was not properly forced into the joints and that voids in the grout are an installation fault created by the tradesman.
Recommended procedure for repairing voids in the grout
Voids should be completely exposed and then filled with fresh grout.