When I post, there are 5 lines below the message box, called Options: HTML, BBcode, smilies, etc. The last one is Notify me when a response is posted. It is automatically checked because I have that preference set, also.
Is you email addy correct in your profile? Could your email be weeding your notices as spam? Is your email set to check for new mesages frequently while you are online? That's all I know to check.
I'm sure sean can help, or another mod.
HTH (not the pool chemicals! 8) )