[Zubersoft]

There are two lock settings in MobileSheetsPro - the Device Orientation Lock already mentioned and the per-song orientation lock. Assuming both are off, the app absolutely should let you rotate into any orientation you want. I just verified on my Samsung Note Pro 12.2 that I can rotate the tablet 180 degrees without issue. Have you tried dragging down the top of the screen and toggling on/off the screen orientation setting? This seems to me to be some kind of OS setting on your device that needs to be adjusted.

Mike