AdMob for Android

I would just like to know, if I have done "show ad" on one scene, will i need to do it again when I switch scenes, or does "show ad" keep the ads up til the app is closed?

EDIT: I found a weird glitch, when I open my app, if i leave it open the AdMob Ad will not show, however if I switch to multitask and go back to the app, the Ad is there. Not really sure how to deal with this

I would just like to know, if I have done "show ad" on one scene, will i need to do it again when I switch scenes, or does "show ad" keep the ads up til the app is closed?

EDIT: I found a weird glitch, when I open my app, if i leave it open the AdMob Ad will not show, however if I switch to multitask and go back to the app, the Ad is there. Not really sure how to deal with this

Just use  "show ad" in the first scene. It will show in every scene. Only "Show ad" again after you use "Hide ad"

EDIT:
You have to override Back Button on android, this will disable the default back button behavior on Android.
Go to:
Game Settings/Mobile/User Input
Enable "Override Back Button"

Thanks for your reply, however the Ad still does not show when I open the app, it is only when I double click my home button (open multitask) and go back to the app, do i see the ad, I think the ad is working but is invisible when the app is first opened