The most common reason you'll see a blank space on your site is that you're logged into your site. Being logged in to your site while you're browsing can create caching issues with your ads. It's not a problem, it just means your ads will appear blank to you while you're browsing. You can test this by logging out of your site and refreshing the page. Your ads should begin appearing.
Sometimes an advertiser will have a code error with their ad so it shows up blank. Don't worry, you are still getting paid for that ad impression, and because of that, advertisers are usually quick to find the error and get it fixed!