I created this head by mirroring the right side. I didn't know how to merge the mesh but when I checked the Merge option in the Mirror Modifier, as I was prompted when I typed the title, I was able to grab all of the mesh. Then I could mark the seams for the UV map. When I unwrapped the mesh as a UV map, it appears that only half of the mesh unwrapped. I think I made the mirrored portion of the mesh of the head a part of the whole so what did I miss, and how can I fix the unwrapped mesh?
It's hard to tell for sure, but here's my best guess at what happened:
So if this is the case for your object, then when you thought you were UV-unwrapping both sides, you actually weren't.
If you apply the modifier you will be able to unwrap the whole mesh.
By the way, all 'Merge' does is seal the seam at the center of symmetry, by treating each mirrored vert pair as a single vert in the modifier result.