Somehow, there is too much flex in the shower pan, so as people move around on it, it is popping up the grout and tile. If a pre-mixed thinset was used on the shower pan, it won't cure properly, so perhaps this was the issue. Or, perhaps not enough polyurethane sealant was used to prevent the pan from shifting. Also, the clamping ring has to be secure around the drain. Professional installers use a heavy box of tile in the pan while the thinset sets up for a solid base. You may have to start over.