subfloor under tile moves/flexs causing grout to crack. Is there a flexible grout on the market or an additive I can put in the grout to make it more flexable. I can not remove the tile as I can not find any replacements for it (It took 2 years to find what we like and it is no longer available that I can find. I tried using straight latex caulk but it was messy and just would not level.