Hi firstly I have to say I love OGraphr - at first I didn't because of this problem but then I realised what the issue was then was very happy my video posts now have the proper thumbnail!
The issue is that my theme (Shaken Grid) like some others has a custom post type, a separate 'Media URL' field which creates the auto embed for you (this was before WP included them) and appears above the post title like with Featured Images.
This means if you post the video there, no thumbnails are seen by OGraphr at all. Only way around this is to also post the YouTube/video URL into the main post, save it, then delete it, which will be a pain if I forget.
Is there a way that OGraphr could check a custom field also?