Android Question FailedToReceiveAd Reasons (Admob)

Hello,

I try to understand when the "FailedToReceiveAd" sub in Admob is called.
I think that it is called when there is no internet connection. But could there be any other reason?

Things I've noticed:

• On my application it's called in about 20% of cases.
• it's not related to a device category, screen size or android version, because I get the ad_received called on same devices categories, screen sizes and Android versions.
• I don't think it's related to the fill rate, because my fill rate is very close to 100%.

Screenshot from Google Analytics showing Admob's 3 subs percentage:


So, does the above graph says that in 20.6% of the cases, my users have no internet connection?

Thank you.
Jmon.