I note that your selection of vertices include some (in the head area) that are not actually on your line of symmetry. That can't be good.
Max is good about giving you a legitimate alignment of vertices from the front view. Sometimes if I try in other views it gives me unpredictable results. So, I'd clean up the selection so that only vertices on the actual symmetry axis are selected, go to front view (set the dropdown box to "world" or "view") and then select the "x" button on the alignment panel
I made a simple example, my silly box is a jaggedy editpoly object. I make a copy of the box, and select ONLY those verts I want aligned. Then I press "x" in the next image. Third image, the desired outcome.
