Crack in newly fitted travertine tile. What would cause this?

[WordBear]

Tiling nightmare continues...

These were fixed at the start of the month and in the last few days this crack has appeared. I think a crack (or two) might be appearing in the tile above too.

We're not in a good place with this. I've no confidence in our tiler's ability (see previous thread) and I doubt he has insurance. We haven't paid him fully for the job yet because he messed up the layout and is supposed to be back in a couple of weeks to replace 28 of the tiles.

Any suggestions as to the cause?

 

[WordBear]

This is my previous thread: Help, I think my tiler is doing a bad job. What should I do? - https://www.tilersforums.com/threads/help-i-think-my-tiler-is-doing-a-bad-job-what-should-i-do.85628/

To bring things up to date: I tried to find a different tiler to finish the job but couldn't find anyone who wanted the job / was available. Let the original tiler finish the job. He has fixed / is fixing some of the issues. Crap cuts are covered by grout and Silicon sealant.

 

[WordBear]

Forgot to say that the tile that is cracked was fitted to replace a tile that had been badly cut around the shower tray. I'm not sure if that could be a factor.

 

[Bill]

without knowing the full facts........natural stone can be extremely fragile and also this tile could have already been cracked without the tiler knowing.

 

[WetSaw]

As above or movement in the substrate ( joint in the board ) , big blobs of adhesive drying out.

 

[WordBear]

So perhaps not his fault? It is very close to a joint in the board (tile backer board fixed to masonry). I've looked again at it today and I'm pretty sure the tile above it is cracking too. You can see better in this photo taken in daylight:

That one, the cracked one and one next to it to the right also sound hollow.

I know the adhesive he used here wasn't marked suitable for natural stone. I know that there are gaps in the adhesive (it isn't dot and dab he just applies it sparingly) and that he didn't butter the tiles with adhesive. It's also a 2mm grout gap which I read was too small for natural stone.
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I don't know whether to get him back to fix this and pay him the rest of what we owe him or tell him not to bother coming back on the basis that it looks like he has messed things up so badly they're all going to crack and fall off.

 

[WordBear]

I forgot to say that I don't think he taped the joint that runs under this tile.

 

[Simon Dale]

2mm is standard. You should all ways back butter