It usually takes a few seconds for the ad plan to be loaded from server. You should at least call IsReady() to verify that ads are ready to show, in case you are showing ads right at the beginning of your game.
Have you enabled “Change video orientation according to device” for your zone in Ads Admin dashboard (https://unityads.unity3d.com/admin/#/games/?))? Also check if you have locked rotation on the device.
Regarding zones, please take a look in the Admin dashboard. In short it’s possible to have multiple zones, some non-rewarded (i.e. user can skip video) and rewarded (user cannot skip)
Please see Unity Blog and Unity Blog for more information and recommendations on how to integrate ads into your game.