Why are my tiles flaking?

[Clueless]

I moved into my house in January, and since then, some of the tiles in the shower have been flaking away. Photos are attached.

Can anyone offer any guidance on what might be going on and what I should do? Could it be a sealant issue?

They are stone, but I don't know anything about the material, installation, etc. beyond that. There is under floor heating (and the bathroom floor is higher than the hall outside as a result). We haven't done anything unusual - just showering with hot water and regular shampoo and body wash.

 

[Dave]

The flakey skin parts is sealer and requires stripping back and re-sealing. The shelled tile is basically slate is a sedimentary stone and can split away layers. When you re-seal it’ll seal those parts as well.