Part of rigged mesh doesn't move with the rig I created this mesh, then made a rig for it and did everything right with the IK rigging.  

As I go to test it I turned the head and half of the face doesn't move with it. I kept trying a multitude of things to get it to move but it just stays there. Can anyone Help me?  

Things I have tried Re-rigging it and reapplying the mirror modifier.
If some vertices are not influenced by the movement of the deformation bone probably the don't have enough weight assigned. In Object Mode select the bone first than the mesh, change to Weight Paint and add more weight to the vertices (blue is completly unbiased).