Sam,

This can occur when the owner of those sites have slow unreliable servers or use some flash/javascript objects that causes the page to take longer than usual to render, and in turn causes our thumbshot bots to capture the background (white or black usually) only. In general, webmasters should try to make their pages bot friendly when possible. You can think of the same problem with search engine bots (like Google or Yahoo) that may not be able to index a web page accurately if a page contains a lot of complex javascripts, heavy flash objects, etc..

We can submit those sites to recapture and see if they work or the owner of those sites can try to improve the render time and reliability of their pages.

Thanks