Have a wood subfloor and tile in the kitchen with the grout cracking or coming out ? Have you concidered having the grout removed and regrouted ? Since grout does get dirty and over time will turn a different color in traffic areas . It may be best to completely remove the grout and regrout the entire floor so that the color wil stay consistant . Take this foe slate 18 x 18 Stagger pattern for instance . The wood subfloor was moving a little and since was supported so that the moing was stopped.