Hi,
We installed tile over tile in March. We used a rubber adhesive thin set six. Over the last few weeks we have seen several cracks in grout and a few tiles that appear to be loose. The humidity the last few weeks in our area has been crazy. There is movement and crunching as you walk on them. And we have discovered a few more today!!
What can we do to fix these problems? Our concern is the entire floor we start coming up? Appreciate any advice! Thk you..

Unfortunately, and I hate to say this, but you might have to take up the new floor that you have put over the old one and put a new one over it again with a proper underlayment of some kind. Depending on the type of tile that you are wanting to put down, there are a number of different kinds of underlayments that you might use.

Please advise us as to the kind of new tile it is that you are putting down.

However, I wouldn't recommend putting tile over tile, as old tile is not a proper underlayment for new tile of any kind and putting new tile over old, no matter what kind of tile it is, as doing that will most likely void any warranties that the company that makes the new tile will provide.

I'm sure that others will come along to explain this better than I can. But, I have done some research about this because I have had this type of situation come up with customers wanting new floors that they are wanting me to install where the old tile will not be removed in order to install the new tile.

It sounds like the adhesive did not bond to the old tile surface. I agree with Clough.
I think you need to remove both layers of tile and start from scratch with 1/2" backer board.