The answer is simple. Google AdSense uses Javascript. If Javascript is disabled, Google AdSense ads will not display. The solution is to check to see if Javascript is enabled or not, and if not, enable it. If it is already enabled, but you still can't see it displayed, then refer to AdSense Support. I myself have had similar problems like that myself.
Enable Javascript for FireFox browser
For FireFox browser, what you need to do is to click TOOLS > OPTIONS in the menu bar at the top of the browser. You will get a "pop-up". Click the CONTENT tab and you should see what is shown below:The CONTENT tab and the radio button to enable or disable Javascript are highlighted with red circles. If you want Google AdSense to be displayed in FireFox (click BACK button to get back to this page), make sure the radio button for "Enable Javascript" is ticked. If not, click on the radio button to tick it and click OK to save the setting.
Internet Explorer
This will be added laterFlock Browser
The Flock browser is based on the Mozilla FireFox browser. I use multiple browsers for my work and often have a few browsers opened at the same time. For reasons, refer to Why you should have more than one web browsers. I started using Flock when I could not sign into Google Blogger with the updated Opera. See Problems signing into Google Blogger with Opera browser.How to enable Javascript for the Flock browser will be added later.