Thanks Jammy, have you verified that adverts are actually being displayed in your app? Do you mind me asking what app it is?
I check AdMob almost hourly and although revenue/clicks can take 24-48 hours, requests/impressions are normally pretty instant. The strange thing is that requests are still going up, its just impressions which are 0. I've even checked my app and no ads are being displayed, something is definitely very wrong...
EDIT: Here's a screen shot of my last 7 days... As you can see from yesterday, impression count just stopped at 7271, from then, we have not received a single impression. Pretty strange seeing as we normally average at 99+ Fill Rate...