How notification emails work
If you participate in a topic you will only receive an email for the next reply/activity in that topic, and even if there are more replies you will not receive any further notification emails about this topic until you visit that topic again.
After you have visited the topic again (while being logged in) the system will reset and send a notification again for the next new activity. This is to prevent you from getting continuous notifications for a topic you may not be interested in following anymore. This default setting cannot be changed.