If you’re not getting ads and your log cat log/device console logs are outputting this message: “MOPUB: Banner ad view is fetching ad network type: clear,” your ad unit is not warm, and the ad unit has been pushed off the server cache of the specific ad server the ad request was sent to.
Each of these ad units have to be warmed up on all servers to have ads deliver properly. Because each ad request will propagate the ad unit across a number of servers, it takes a nominal number of requests from a specific ad unit to make sure that every server is warmed up for that ad unit.
When your app is live, this issue should not occur, as enough requests will be made to keep the ad unit live on the server cache. Each ad request propagates the ad unit across a number of servers such that each subsequent ad request on that server serves the ad, given that the ad unit is still on the server cache.
One way to manually get these ad units to start serving again is to edit the ad unit. This will force a refresh on our servers within 20 minutes and push the ad unit across our servers.
Additionally, newly created ad units will be automatically warmed up for all ad servers for a period of 48 hours.