From the On-Page Analysis help:
"The On-page summary automatically generates reports for any of your campaign keywords that rank in the top 50 of your primary search engine. The URL that it grades is the same URL that appears in the search results.
For instance, if you have 75 keywords ranking in the Top 50, you should have 75 On-Page Reports. This generation happens automatically within 24 hours of when your rankings are updated."
You can find the help here:
SO, in answer to your first question - are any of your pages ranking in the top 50 for any of your keywords?
You can also run an On-Page Analysis on any page against any keyword by using the tool from the "Research Tools" menu. It's here:
Hope this helps.