Humm my last suggestion would be for you to go to your profile, "notifications and email" and uncheck one option that is on by default: "Receive reply notification only for the first unread reply."
According to the manual: "Receive Reply notification only for the first unread reply - Checking this will only send you a notification of the first unread Reply instead of every Reply made."
This may be the cause of the problem you described (hope so!

)
edit: i guess what you want might be something like what
this addon does. It surely creates a lot of spam for big forums with good activity so i guess that i understand why it is not something available by default. I may give it a try one of these days so you can get more spam

edit2: other alternative, that i don't know if you're aware of, might be RSS feeds. Not exactly the same but it could be a solution.
They are active on our forums, even if the layout sucks a bit and members are not aware about it (i should do some changes one day and find the reason for some bugs). If anyone cares, feeds can be used for a specific board or the entire forum, for example:
http://www.tosecdev.org/index.php/forum/index.php?action=.xml&type=rss2&limit=10