Sometimes this is due to an ad blocker being turned on in your browser. Please check your browser settings to make sure that an ad blocker is not turned on and then try again.
If you are sure that your ad blocker is turned off, then it could also be that certain code surrounding your ad code is causing the div to be hidden in a particular browser. Browsers can render code in different ways, and some browsers are more sensitive than others. If there is an error in your code, or if a certain CSS property is being used, it may cause display issues in some browsers.
You can use the W3C Markup Validator to check your site for code errors. Sometimes correcting code errors can also correct any ad display issues.
If you need further assistance, please open a support ticket so we can help you resolve the issue.