Sometimes the eyes stop tracking to the mirrors... for me this happens when I import a project someone else made. Simple solution:
In your reflection scene, select the "plane" object, and add a "reflection object" script to it
Go to your character. Find the property "Human Auto Attention". Under it, find the property mirror. If your eyes aren't tracking, this probably does not have an object. Drag your "plane" object (the one you just dropped "reflectionObject" onto) and drop it into the "mirror" slot.
Make sure whatever axis is pointing up is set in the mirror property as the Mirror Up Axis... and whatever axis is pointing towards the character is setup as the Mirror Normal Axis.
For reference, X is red, Y is green, Z is blue.