Having unused impressions means that we are trying to display your
banners that many times. Unless your account is "on hold," you don't
need to do anything. Our server will continue to try to find suitable
sites to display your banner.
We will not display your banner if your account is "on hold" --
see your status on the upper right corner of this page. If "on hold,"
click on the "Reasons" link below it to go to the Memo Desk page where
you can try to resolve the problem.
Normally, you receive two unused impressions for every three
banners that your site displays -- hence our minimum exchange ratio of
2-to-3 (or 0.67). But when you have active hit credits, our server
will also periodically give your account more unused impressions,
thereby raising your exchange ratio.
If the unused impressions are building up and your account is not
"on hold," then your target categories are probably set too narrow.
Try adding more targets or clear them altogether, so your banner could
be displayed at more sites.