Its not a problem persay.

There is going to be a percentage of your visitors using an Ad Blocker or have javascript disabled on their browser. So when these guys hit your site, they don't see the Google Ads.

Then you have the happy clickers hitting the site. These are the ones who bounce through and click on the first thing they see and its usually done before the whole page loads, including the adsense.

Another consideration is that there isn't enough advertisers in the ads pool for the publisher. When this happens you'll get less adverts showing on a page.