Hi, can you please tell me which forum software are you using for your forum? Also some forums send notifications about new replies only after the user (notification receiver) had visited the forum, if they don't visit the forum then no more notifications are send.
I have +4 years experience modding and customizing many forum softwares, including SMF, phpBB, vBulletin, Xenforo, MyBB and others.
If you have any question just let me know.
Greetings!