Check your website to make sure that the Google Adsense code is being displayed (e.g. 'view source', and search for the google tag).
If it is, it is likely that Google Adsense is choosing not to display the ad. Usually this is because there are too many Google ads on a page, or google decided that there is some reason that you are not adhering to their terms and conditions (e.g. competitive ads, too many ads per page, etc.)
If the adsense code is not in your page, then it is likely that you have rules in Advertising Manager which restricts the ad to display on the blog. For example, if the ad should display for a particular author, but on your site the article is not by that author.