I am having the same issue with the Ajax plugin in. It looks to be when wordpress requires the user to input a captcha math problem on the wp-admin side.
Richard is the the same issue with Google +.
At least facebook there is a work around of just posting the link directly and choosing the picture. Google plus always defaults to my banner.
I am having the same issue. The only work around I have found is pasting the link in the subject and manually selecting the image I want on facebook.
Do you have the same issue when you use google+?
