I just joined Stack Overflow as well, and this is my first answer :)
I too have had difficulty with AdMob, and I couldn't see any ads after a few days. My best guess as to why ads were not showing up was because after looking at my webapp through a desktop browser so many times, the people at AdMob flagged it as 'not mobile worthy' or something. I had a fill rate of about 8%.
Another problem may be that your page doesn't have enough views.
My suggestion is to, if possible, use a different ad service, such as AdSense. After having the same problem you're having and not getting a straight answer, I gave up and made some fancy (and legal) workarounds to get AdSense for my mobile apps.
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