Try This...
Go to your notification area in the taskbar.
Right click on Bluestacks Agent>Rotate Portrait Apps>Enabled.
There are several options available..
a. Automatic - Selected By Default - It will rotate the app player in portrait mode for portrait apps.
b. Disabled - It will force the portrait apps to work in landscape mode.
c. Enabled - It will force the portrait apps to work in portrait mode only.
This May help you..