Hi there. Could you please provide additional information about what happens when someone tries to comment on your blog and fails? If you'd like, you can forward our email to them (ticket #1397891) and ask them to provide information directly to us. For example, are they seeing an error message? Are they being asked to log in? Do they leave a comment but it never shows up on your blog, not even in the spam queue? (In that case, it's possible that our spam engine, Akismet, is catching their comments by mistake and they should fill out the form here: http://akismet.com/contact/)
Once we know what is blocking them from leaving a comment, we'll be able to troubleshoot further. Thanks in advance for providing additional details!