It seems that a production error has occurred with the First Edition Voyager Optimus toys (this one - both Hasbro and JP versions), in which the head has been assembled over a ball joint without actually popping the ball into the socket inside the head.
As such, the head has very little movement, and can't even look up or down.

A fix has been posted up here, which says to unscrew the head (single screw on the back), remove the front half of the head... pop the back half of the head onto the ball joint, then screw the head back together.

I just did it, and it is a noticeable improvement... and I can see how it was missed at the factory, as the head had a cavity in front of the socket that was just big enough to hold onto the ball joint, so would have been assumed to be constructed properly because the head wasn't falling off.