This is the same issue I dealt with for the last year and a half. See this thread.
For myself it is now working since they modified the data packet size on the server cluster our managed wordpress site is hosted on. Reference tech support to this thread and ask them if they have done the same on your site's server cluster.
If they have and it still isn't working then pretty much you are out of options. I had a year and half of misery before they fixed this issue for me.