No thumbnails when posting to Facebook and Google

Hello,

I’ve been experiencing intermittent difficulty for months in getting thumbnails to appear when I post to Facebook from my site. Now I am completely unable to get thumbnails when posting to Facebook or Google+. I’ve tried the Facebook debugger (https://developers.facebook.com/tools/debug) and two WordPress plugins (Fedmich’s Facebook Open Graph Meta and Easy Facebook Share Thumbnail), neither of which worked for me. I also have a ‘Featured Image’ set for every post.

Regarding the plugins, are these supposed to just “fix” posts going forward, or all posts going back, as well? Is there another plugin I should try?

I contribute to 2 WordPress sites, one of which shows thumbnails in Facebook and Google+ no problem. The site I’m having trouble with is https://solitarywatch.com (News Child Theme with 18 Widgets).

What causes this problem to begin with?

Thank you in advance for any insights or tips on this!