Flooring Tile - popping grout

I was talking to a man at church last night and he was explaining the problems he is having with the grout on his kitchen floor. He and a friend screwwed durarock on the floor and installed 12" tiles about 6 months ago. 24 hrs after grouting a bunch of the grout popped up. He regrouted the next weekend - same result. he has since went under the kitchen, checked and beefed up any floor joist that looked suspect, regrouted - same result. I told him using latex additive might help but I'm no tile man. Any suggestions?

Need alot more info., Like, joist size, spacing and length of them unsupported, subfloor under the Durock, how many layers of what and how thick, type of grout used and how was the Durock installed, get the picture.:)

Additives won't solve a thing here, there's alot of movement somewhere if grout is coming up that soon, even with some movement, it shouldn't fail that fast, somethings not right here.:)

How did he install the grout and what brand was it, joint size and was he using sanded or unsanded, bet your sorry you asked. :D

Not sorry I asked - gotta start somewhere.
I assume he used sanded grout since it is floor tile. I'll try and get more info Wed nite
thanks

Mark, make sure when you ask how the Durock was installed, Tileman wants to know "if" a bed of thinset is "underneath" the Durock, full spread, troweled out with a 1/4 x 1/4 sq. notch trowel . Was the Durock cross laid and staggered over the subfloor seams leaving an 1/8" gap, and were the Durock joints "taped" before laying the tile.

Talked to him again tonight. All the wonderboard was installed correctly. He did admit that the joist where undersized [for the length] and that it actually takes a week for the grout to pop.

We discussed him erecting a beam to support the load under the middle of the joist span. I believe that should adequately support the floor and solve the grout problem.

thank you both for your responses :)