Bixley Tiling

I've had the same problem with grey Bal flexi grout in one area near a back door recently.

Used a very fine sandpaper lightly on it and it took the mottled look away but i wasn't totally happy so re-did the whole area. It was a lot cooler when i re-grouted and it has dried up fine this time.

Does it happen more often if the grout dries too quickly??

Mark.