A: Admob displaying three preconditions:
1 It needs 3G network.
2 Everytime you flip page, it sends a requirment to Admob which can be search on Admob homepage (it isn't real-time, always you can find the statistic after several hours).
3 The requirement of Admob not all the time displays, so maybe alfter several flipping, it can come up. (Samely you can also find them after several hours on Admob website).