Grout cracking issue

Ok guys i have a customer with a sun room and his floor has a little to much flex for my likeing and his grout has started to lift and crack. I didnt lay the tile or build the sun room but he wants me to fix it. Problem is he put the tile down. He used Ditra ontop of 3/8th's which is not enough sub floor in my mind and this is the cause of the issue. Other than taking up the hole floor and beefing up the subfloor if there another way to repair the grout. Maybe a more flexible grout??