I think I know the problem… ShrinkTheWeb.com has changed their model for non-paying customers to not allow for local caching of thumbnails they generate.
Something to do with that. I’m not a techy-
I solved the problem by going into the advanced settings, and put in “custom thumb URL” for all 17 of the sites in my portfolio, using an image from each site. It was kind of a bitch, but I don’t like waiting around. I actually think it looks better anyway.
It’s not so much a problem solver as a problem work around.
Good luck- and hopefully the team at WP Portfolio will fix the problem.