Cracked floor grout

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I have some terrazzo tiles 600 x 600 that were laid in February this year onto sand and cement screed (I had a whole saga that last about a year, which you can read about here). The tiles are quite thick, I think around 18mm.

Most of the grout lines are fine. However there are a few tiles where the grout joins have cracked. This is mostly, but not exclusively, in a patch of floor that was tiled later, around June, because we were waiting for an external door to be fitted. 

 

We also have some external patio tiles, similar dimensions but made of porcelain, that were tiled in July. They are also mostly fine, with the exception of one or two tiles where the grout has cracked and now there are bits of grout missing (we had a thunderstorm the other night, so maybe the chips were blown off by the wind?).

 

I'm just wondering what the remedy is here. Should my tiler come back, remove the grout that is broken and re-grout? What if the problem is not the grout but the way the tile was installed (i.e. not enough adhesive)?

 

Both grouts are grey, but I can't remember the colour. The internal one was Fugabella and the external was a Maipei.