That's a sign of too much deflection (movement) in the floor.

Do you have any leftover grout from that tile job? It's not a perfect solution, but you could clean out the cracked grout lines and regrout those parts.

Won't help. It'll just crack again.

How long did it take to crack, and how extensive are they?

Bill, any "grout-like" products out there that claim to be more tolerant for situations like these? Short of redoing the whole thing, how do folks deal with this? ( Perhaps dark grout will help to hide the cracks....just a thought)

Well, there IS a product that won't crack, and that's epoxy grout. However, think about these givens:

Structural movement occurs in all homes and must be accounted for.

Tiled surfaces (ceramic, porcelain or stone) are rigid, hard surfaces.

Movement not addressed WILL transmit through rigid floors at the weakest point.

Now, if you grout with epoxy, the grout is no longer the weakest point. Guess what's going to crack NOW?

The only way to solve the problem, is to locate the source of the movement, and address it. In the case of too much deflection, that might mean adding cross blocking or sistering joists under the floor. Or, you could add walls or bracing under the joists that exist. One way or the other, either the joists need to be reiforced, or the span cut down.