Check the destination directly
If a campaign area is blank, use the available Visit Website/Open Advertiser action to test the destination in a normal browser tab.
A common cause is the destination website's own iframe policy. Headers such as X-Frame-Options or CSP frame-ancestors can tell browsers not to embed the site.
If you own the destination, adjust its framing policy only if doing so is appropriate for your security model. If you do not own the website, the traffic-exchange platform cannot bypass those headers.