tony TLC

2 questions is the was the floor stable when they started laying as in not floor boards and not chipboard (lots of new floors have a lot of movement)
are the tiles thin if so there wont be that much of a key for the grout to hold to if you have movement in floor
i have fixed a problem like this (not my job i must say)i removed all old grout and used a ready mixed fix and grout (timberflex)and this orked for me you didnt say what colour grout you had?