Date: 11/13/2008 3:06 pm EDT
This is a puzzler. Without seeing it first hand, its difficult to say exactly what happened. I would guess it was possibly "overwashed" in that area; e.g., there was too much water introduced through the cleaning process. While it is important to "clean as you go", using excessive water can cause curing issues. Not saying that is what happened, but a guess.
The grout is designed to adhere to itself, so I'd suggest scraping out the 5 ft and regrouting that area. Be extra careful to not introduce too much water during the cleaning process.
If possible, I'd really like to get some samples of the grout that didn't cure properly so we can work with our grout manufacturer to determine root cause. Our address is Avaire, 13423 "F" St, Omaha, NE, 68137. You can make the package to "Jonathan's attention".