Originally Posted by CyberSEO

You are using the CyberSEO plugin, right? If so, just go to: CyberSEO->Tools, select "Generate missing post thumbnails" and click "Apply These Modifications To All Existing Posts". That's all you have to do

BTW, I was found your question here almost accidentally. Please don't hesitate to ask your CyberSEO-related on the official support forum.