Can you let us know what site you're seeing this behavior on so we can see and test further? Also, please note that automatic login will only trigger when the user first comes to your site. If the user has specifically logged out of your site, they won't be automatically logged in for a period of time. So, there are cases where automatic login
won't work, but that's actually intentional.
I'd recommend trying to clear your browser cookies, close the browser, then start over: Go to FB.com, login, and then go to your site to see if the auto-login happens.
Finally, if you haven't upgraded to JFBConnect v5.0 yet, we'd recommend that.
Thanks,
Alex