A few questions here;
Have you tried using straight chlorine bleach to remove the stains? and
When you regrouted, did you take the time to remove the existing grout? (a tedious task, to be sure, but probably nessisary.)
When you regrouted, did you seal the new grout?
There are others herw who probably have more knowledge on this than I do. I have seen grout being replaced on kitchen counters and the old grout was always removed first.
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