One eye of model doesn't follow head? One eye of model doesn't follow head?

I have a bit of a problem with a "4 Piece Model", "Head, Body, Eyes, Hair, and Teeth". I hope someone can be of assistance while the "Eye's" appear as a pair and don't act as a pair. Upon selecting the eyes in object mode they both highlight as a pair with a single click of one eye and then after shift selecting the models head and choosing Ctrl P > "BONE" the parent only applies to one eye and the left eye doesn't move with the head and rig movements? There is no lattice or such, or any such tracking.
Your eyes are incorrectly weight painted. To fix this, Selected to vertex group DEF-head and go into edit mode (Tab). Select all your vertices (A) and Hit Assign.