Flooring Tile - tiles coming loose

in my bathroom i have installed 12x12 stone tiles on my floor. i took up some 1/2" plywood subflooring that was rotten and replaced with 1/2" cementboard. about 8 months after completion i noticed the grout staring to pop loose. now, 18 months later some of the tiles are loose and the grout is almost gone. i used the same tiles for a 3" baseboard as well the tiles near the walls seem ok. is my floor flexing? bad thinset? thanks in advance.

Yes it is flexing.

What is under the cement board?

thanks for the quick reply.
i believe there was 2 sheets of 1/2" plywood originally. now 1 sheet of 1/2" plywood under the 1 sheet of 1/2" cement board. this is on the first floor with a finished basement with drywall ceiling.
what is odd is...this is the third bathroom that i have done in my house like this. this is the only tiling issue that i've had. i've tiled them all the same way.

One layer of 1/2" is not enough to support a tile floor and a 1/2" CBU does not add structural strength.

this is wierd. i have a bathroom next to this one with a shared plumbing wall. i installed the tile on that room as well. using the same procedure.

how do i remedy?

Stone requires twice the support as tile. If you used that method in the other bath with ceramic and it's holding, count your blessings. At a minimum, you would have needed 5/8 t-g plywood subfloor with 1/2" backer and that is scraping the barest bones minimum. That would be for ceramic, not stone. If you really have only 1/2" of subfloor, you are now just playing a waiting game for probable future issues with the other floors.

To fix the issue with the stone, you will need to pull the floor up, add 3/4 bc graded ply then backer. You will probably need ti reinforce your joists as well. What's their size, spacing, and unsupported span?

the other bath is actually 12x12 granite tiles i installed in 2000.

the hall bath floor is about 6.5'x4.5'. it's just a hallway bathroom. i used the same tile as baseboard and as the tub suround so tearing it out is gonna make me sick. the joists underneath are on2x8 on 16" on center. therei s a beam that runs the length of house and is pretty much under the door of that bath. the house is a raised ranch.

thanks again for all the help guys.

I see two problems with your install.

1. too much deflection from your floor joists. Stone floors require a deflection rating of L720 or better.

2. Not enough subfloor.

While your other tilework seems to be holding up well for now, chances are it will not last the 50+ years that it should.