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@lynx I would hack out as much of the cracking grout as possible and get a good wide joint grout and start again - BAL and Mapei both do wide joint grouts, they tend to be the industry best so I would stick with them.
Just make sure you mix it well by hand and stick to the ratios they advise, work it into the joint for as long as you can, by going over it with your grouting float to get any air out of the joints which could cause it to go off quicker than intended. Best of luck mate
@lynx - wouldn't pay any attention to Mapei saying it needed to be mixed mechanically. I have always mixed by hand with no issues.
I'd be inclined to try a specific wide joint grout. BAL do a wide joint grout here: https://www.tilefixdirect.com/product/BAL-Wide-Joint-Grout?gclid=CjwKEAiAo7C2BRDgqODGq5r38DsSJAAv7dTPPC5bZFjgXRtHh56yyR4XbjsIJD_wzq96SwAzJ8ZkGxoC8XDw_wcB#BAWJG10
I've not used BAL myself and I have never had a problem with Mapei - it may have just been the mix ratio that caused it to dry to quickly or something. Anyway, BAL gets rated by a lot of trade folk so I'd give that a whirl. Good luck mate