The number of impressions being recorded for advertisers, campaigns and banners and/or websites and zones are significantly lower than the number of requests.
As soon as Revive Adserver has determined which banner will be displayed, this is logged as a request.
When using an Image Tag, No Cookie Image Tag or XML-RPC Tag, Revive Adserver will also log the impression at this point, as there is no other way for these tags to record impressions.
However, when using any other kind of Zone Tag, or when using the website-level Single Page Call Tag, then Revive Adserver uses a "beacon" to record the impression - that is, after the banner has been loaded, an invisible element is loaded in the page which calls back to Revive Adserver to let it know that the banner has been successfully loaded in the web browser.
This is done to provide a more accurate record of the number of banners that are actually loaded (i.e. impressions) vs. the number of requests to load banners (i.e. requests).
If you are using Single Page Call Tags, review your use of the tags. By default, Revive Adserver will load a banner for every zone in the website, even if you're then only displaying some of the zones on the page. This can lead to a high number of requests vs. impressions.
To limit the zones that an SPC Tag will select a banner for, simply add the following code to your page above the SPC Tag:
In the above case, this will limit the SPC tag to simply loading the zones with the name/ID pairs shown.
Probably the most common reason for a significant difference in requests vs. impressions, however, is banner loading performance. Any banner that takes a long time to load, relative to the rest of the page, is likely to significantly increase the "drop off" rate - that is, the rate of users who visit a website, which results in banner requests being logged, but who then "navigate away" to another page (on the site, or elsewhere) before some (or all) of the banners have actually loaded.
In particular, you should consider looking for:
Using a tool such as YSlow, Firebug or one of the many others that provide an analysis of the total page load time can often help you to see and understand the difference in performance between the time required for the web page to load vs. the time required for all the banners to load.
Note: This issue can be particularly exacerbated by the SPC and the Asyncrhonous Tags
If you're not selecting banners via an SPC Tag for zones that you never display, and the load performance of your banner creatives is reasonable relative to the load performance of your page, you will need to review the performance of your Revive Adserver server performance.
Poor performance on the Revive Adserver server (in general terms - not having enough memory, CPU or disk I/O performance to meet the load demands of the server under load) can result in poor banner creative load time and/or lost data, as the server fails to respond to e.g. impression beacon requests, and users' web browsers time out. This can also lead to "missing" impressions when compared with the request levels.