My iPhone 5 won't turn sideways when I'm on the Youtube app?

It works on Safari. I can turn it sideways there and it works but on the Youtube app it won't turn sideways. It just randomly stopped working. Before you answer please note that I did try Portrait Orientation lock on and off but it still didn't work.

Ok it's not like a big problem but before you go to YouTube make sure your mute is off and then go on youtube
<<do not put it full screen yet until you make sure that it turn sideways >>