I had the GD come to our house to replace cracking grout in our shower. Within three months, the grout was cracking again and we were having the same issues. As another reader, the GD did not return my calls until I started to leave somewhat angry/rude messages at the main office. He advised that we have a problem with our shower if it keeps cracking and there was nothing we could do about it. I have DAP caulk to recaulk the areas that are cracking and that seems to hold for a bit, but it takes time to clean and recaulk especially since you have to keep the shower clean/dry for at least three to four days for it to cure properly.
I have yet to find a good solution, but right now I am just resolved to cleaning, scraping and recaulking.
Good luck!