Just to give a little explanation of the forum, when you start a conversation with another user, the server emails them a notice about it. If they click on the link in the email, it will send them into the conversation. However, if they just come back to the site, they either will need to click on the notification to view it, or open their conversations mailbox. Now for the more difficult part. Since the server also handles all the emails, we've ran into getting blacklisted in the past. So we had to implement a system that will NOT email people if their email bounces back. What this means is if someone fills up their email account and starts sending out bounces, we stop sending to them. They then have to verify their email again on the website. This isn't great for ensuring communication, but getting blacklisted is even worse.
Then there's also the people that change email accounts and never update here. Or the people that have notices turned off...
Sorry for the ramble, but I wanted to explain a few possibilities.
