A NO_FILL error can occur if the app is trying to load an auto-sized ad in a container whose dimensions are WRAP_CONTENT x WRAP_CONTENT. This will cause an Auto Ad Size failure and result in no ads being returned. Generally, if this is a screen-width ad then our recommendation for this case would be to change the AdLayout's layout params dimensions to be MATCH_PARENT x WRAP_CONTENT. Auto Ad Size should not be used if a specific ad size is required. Instructions and guidelines for ad sizes can be found here:
If your app can display test ads with the default app key but not with the app key that you have generated your implementation should be working fine. It can take some time for a new app key to become active. Please confirm that you have entered the your app key correctly and try your app with test ads again.