Cracked Grout - Brand New Tile Floor
I'd guess the subfloor. Tile definitely benefits from having a really solid subfloor. It's extra trouble and expense, but the use of concrete board under tile really helps. I'd guess they laid tile right on the plywood.
Is this a brand new house? If so there's probably a warranty. Hopefully someone else will come out with better ideas, but I'd say it might have to be re-done....good luck.