Do you have the orientation lock engaged? Does the iPad move from portrait to landscape ever?
The orientation lock is either the switch by the volume control or, by double-clicking the Home button and opening up the multi-task bar at the bottom of the screen, then flicking the screen to the right, you'll see a control at the far left hand side at the bottom of the screen.
One of these will unlock the orientation. You can permanently decide which one in the 'Settings' menu.
Tim