Water sensitive sandstone?

Installing a floor of "Pietra Serena" sandstone in units ranging from 36"x36" to 12"x36" all at approx 3/4" in thickness. The floor was wet-set. many of the larger units seemed to twist and cup shortly after installation. I've seen this in green and black marble, but never heard of sandstone beeing reactive to moisture in this way. Anyone else seen this?