if it's cracking it means the substrate is not stable enough. Unfortunately this means that not only will you have to pull up the grout but most likely the tile as well.
Your substrate must be built up to something more rigid.
How bad is it? Is it a spaced joint with sanded grout or a tight joint with unsanded grout?
Fill in the cracks with siliconized grout. It comes in many colors.
I put tile in my Kitchen a couple years ago & some of the grout has cracks , any repair options without digging out the old grout & re- applying it? Thanks
Rob
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