The most common reason for seeing a blank ad space on your site is because 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 can 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 which makes it show up as 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!
If you see this happen often when you are not logged into your site, just submit a request here in the Help Center, and we'll check things out for you.