Ads aren't always spread out evenly. If you don't keep a close eye on the trends, you may find yourself wasting time and money driving traffic to your site when you're unable to fully monetize it. Many people don't understand that ads aren't in infinite supply. Some suppliers may run out of ads after a certain time, meaning that it's possible that there may not be any ads to show to your audience. This results in pockets of time when you're not making any money, regardless of how many visitors your site may have.
For example, it's very likely that one or more of the ad networks you use to monetize your site may exhaust their supply of ads for the day by 5:00 pm, meaning that any traffic you get to your site after that simply won't see any ads from that provider. Visitors could spend hours roaming around your pages, and if they never see an ad, you'll get nothing out of it. Similarly, it's common for ad networks to have fewer ads to show at the end of a month than in the beginning and far more ads in the back half of the year than the front half. (About 70% of all online ads are shown in the third and fourth quarter, between back to school and Christmas.) In fact, for many people who make money primarily through online ads, the first part of the year is nothing more than making sure everything is working and in place so they can capitalize on the second part of the year.
Ad networks can provide you with ads only when they have campaigns to run. To make sure they meet their goals, they spread those campaigns across a large number of sites to reach a larger audience. These campaigns come and go, so be diligent in keeping track of when ads are available so you can minimize waste and maximize profit.