What browser are you using?
I looked at the favorite requests you sent and they seem to be completing successfully, so the pics should be added to your favorites. Something must be going wrong with how the browser is handling the response from the site though. I haven't heard other reports about this, but if anyone else is experiencing this, please chime in.
Doesn't happen any more, but an old problem I used to run into is I would favorite an imagine and hit the back button on mouse which resulted in the image not being favorited. Any chance something like this is happening?
It was during my long stay in the hospital where I kind of missing for a while. Just figured I was backing out before the process completed or something, been years since I've seen it. But literally the only favorite's issue I've ever seen.