There are many types of advertisers.. Some have a lower budget, some have a higher.

The ad company first needs to see the traffic of your site, analyze it and then serve the ads that represent the advertisers best.

For example, a site that gets 100k visits a day, won't show the same ads like a site that gets 10k. common sense. wait, and the ads will be displayed eventually. If they dont, it means your traffic is worthless (like if you get many visitors for India - no offence).