Date: 3/2/2008 6:39 pm EDT
You’ve already answered a couple of questions: you stirred it thoroughly, and you didn’t add anything to the grout. Both good answers.
Let me ask you a couple of other questions:
1) Where did you purchase the Avaire Flexible Grout?
2) Is there a “batch number” or similar anywhere on the grout pail? It would be on a sticker.
We’ve heard of this on a few rare occasions. Usually it happened when the grout wasn’t properly stirred – the lower viscosity portions of the pail can sag in the grout joints, especially in the gaps between the interlocking tabs. The other way it can happen is if the joints are not thoroughly packed with grout; that is, make sure to force the grout all the way into the gaps between the interlocking tabs.
The good news is the Avaire Flexible Grout adheres very well to itself and it is not a problem to go back over the top with another layer in those areas that sank. The bad news is that is additional work – certainly not what you or we had in mind when you started your floor.
Based on what you've described so far, I would recommend waiting 24-48 hours and let everything settle before adding the final layer of Avaire Flexible Grout.