you likely need to turn off "rotation lock". Pull down the menu from the top and there is a little lock with two arrows (I think) around it; just click it. When lock is 'off' it will show a small rectangle with arrows around it, not the lock. My Nexus 7 came with the lock on. Once off, it should rotate when put into landscape mode.
One neat trick is you can unlock->rotate->lock to keep in in landscape mode. So you can lay on your side on the couch and it won't try to reorient itself with "down" relative to gravity.