Sometimes you have to zoom in to3:1 or 4:1 to get enough of a sample for the redeye to work. Also it has to be very red to work, a dark red is a little more difficult.
In a shot of my daughter I took yesterday, I had to increase red saturation in the photo before the right eye would correct. After correction I reset the red saturation. Also, don't center the tool over any catchlight in the eye. Sometimes there just isn't enough info for the tool to see, especially in a group shot, and it just won't work.
What I have done in these cases is open the photo in Photoshop, zoom way in and use the paintbrush in black, adjusted to the size of the pupil with about 30% hardness and dot the red away.
HTH
Rich