Since launch, the Instant Previews team has been keeping an eye on common complaints and problems related to how pages are rendered for Instant Previews.
When we see issues with preview images, they are frequently due to:
- Blocked resources due to a robots.txt entry
- Cloaking: Erroneous content being served to the Googlebot user-agent
- Poor alternative content when Flash is unavailable
To overcome these problems, Google has a new Instant Preview tool in the Labs section of Webmaster Tools.
Here, you can input the URL of any page on your site, then the 'Instant Preview' tool will fetch the page from your site and try to render it both as it would display in Chrome and through our Instant Preview renderer.
Google says -
"Please keep in mind that both of these renders are done using a recent build of Webkit which does not include plugins such as Flash or Silverlight, so it's important to consider the value of providing alternative content for these situations. Alternative content can be helpful to search engines, and visitors to your site without the plugin would benefit as well.
Below the renders, you’ll also see automated feedback on problems our system can detect such as missing or roboted resources."
For further queries and feedback check Webmaster Forum