I had my bathroom tiled recently. The grouting has cracked between the tiles. I tried regrouting it and its done exactly the same thing. Why has it cracked and coming loose? I did leave it long enough to dry.
We would say that the grout is cracking because you did not strengthen the floor and the whole lot is twisting and bending when its walked on. See the project section on laying floor tiles for instructions on how to lay tiles onto a timber floor.
but unfortunately you'll probably have to re-grout the whole lot - unless you are happy to live with the resulting colour variation which is inevitable i'm afraid, even with white grout. unless you are very lucky.