On the website under Manage Settings -> App Settings, the refresh
interval comment says "(12 - 120 seconds)"
Be sure to set it for at least 13 seconds, or the ad/your app will
post an error (not fatal, but obviously not something that you want on
your app).
Can someone from Admob take a look at that? It's not a big deal, but
I'm sure a lot of new Admob users either don't know or will never know
(if they don't check the error logs properly).
-Jim