Is your zone not showing a 100% fill rate?

If a zone does not fill 100% it means that at some point the zone did not render an ad.

The reasons an ad did not render might be that the campaign(s) targeted to that zone might have been paced out for the hour and there were no ads to display. In that case the campaign will be visible again at the start of the next hour.

There may also be targeting (geotargeting or keyword targeting) set on the campaign that prevents ads from rendering in the zone for a given set of pages. This will naturally limit the ability of campaigns to display on certain pages.