In iPhoto 6, I've noticed that at times the red-eye correction seems to stop working. In older versions of iPhoto, you would select the red-eye tool, then select around the area to be corrected with the tool, and it would usually change the color of the appropriate red spots. However in the newer versions, you select the tool first, and then are instructed to click the center of each eye. This generally works, but on occasion it will not work at all, or only change one eye.

The solution is to select the recalcitrant eye BEFORE you click on the red-eye tool. This seems to get all the more resistant red-eyes that I have found so far.

Thanks to briandallas for his answer to this Apple forums discussion on "Removing Red Eye"