Sometime ago, my phone decided to stop changing from portrait to letterbox when I rotated it. Unfortunately I can't nail down the time; all I know is that it was prior to installing 6.1.4.
Ideas, (a fix) would be appreciated.
Thanx,
Do you perhaps have the screen rotation lock turned ON?
Double click your Home button, scroll right to the first set of music controls and tap on the little circular arrow in the box located on the far left. That is the lock/unlock screen rotation button.