I just redid my bathroom shower. All tile, floor to ceiling. I just noticed that the tiles in a straight line from the drain have gotten water under them and are loose. They squish like a sponge (ever so slightly) when you step on them. I am. Totaaly bummed. I used sanded grout and the premixed mortar from home depot. Should I have used a different grout? I sealed the he'll out of the grout and the stone and let it dry for days. Please give me suggestions on what I can do to fix this without completely removing my shower floor. I am on a rock base with 2 layers of backer board, then shower pan liner to special drain, then mortar bed, then tile.

Thanks!

You have me a bit lost on the "rock base with 2 layers of cement board." None of that is a structural base for your pan and if it hasn't already, I would expect it to not be too long before the mud layer over the liner cracked.

The premixed stuff you used to set the bucket specifically states on the pail not to use in wet areas such as shower pans for the exact reason you are experiencing right now. It turns to mush when it gets wet and does not dry out again. It has nothing to do with the grout nor the fact that you applied a sealer. Grout, cement based thinset (bagged) and the setting mud bed will all let water pass right through, thus the need for the liner. If you noticed the little holes around the collar of the clamping ring of the drain assembly, those are to allow that water to drain. Sealers for grout are only to help block stains, but are designed to allow water to pass in both directions.

You have quite a bit of work to do, but I'd like more detail on the base so you only need to do the work once.

Yep. I believe you are a hundre percent right. I never should have used the premixed stuff to create my mortar bed. What type of mortar should I have used as my mortar bed?