We all know the SupportedOrientations property on a windows phone page. Just set the value to Portrait and your App will only support the portrait orientation mode. Set it to Landscape and your App will only support the landscape orientation mode. Set it to PortraitOrLandscape and both are supported. That easy!
By the way, the property Orientation tells the device in which orientation to start.
But, yesterday I set the property on my main page from PortraitOrLandscape to Portrait only. And suddenly also on my other pages, where the SupportedOrientations property was still set to PortraitOrLandscape, the landscape mode was not supported anymore in my emulator. Hmmm, what happend here?
After checking everything for my mistake and not finding any and after a long helpless google session I finaly found my mistake by accident.
I used the pc keyboard (by hitting PageUp). This disables the rotation in the emulator and that was my only problem.
I hope this helps.