Check the browser, browser version and operating system you are using. The Rich experience is only available on browsers that support it, so you may need to update to the latest version. Alternatively, try another computer in the environment with an up-to-date browser.

An override is available to “force” the Rich UI, which you learn more about via this KB article. Note that if you use this, we do not support any unexpected behavior.
Troubleshooting Mail Archiver