Upon initial creation of the Ad (In the Adbrite website), you should have the option to select which type of ads you wish to display, "Banner ads/text ads, Full-Page ads, or In-line Ads". Simply make sure that you have full-page ads selected and they should be displayed. If you do not wish to create a new ad, simply select, "Ad specs" within the pre-existing Ad zone you wish to view and reselect full-page ads.

Also, by default the full-page ads are shown 3 times ever 24 hours. You may wish to change this to either 6 or 12 for higher return.

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Cheers,