when at the bottom of the thread there are 2 reply buttons.
- the top reply link replies to the poster who posted that comment and
notifies them that there is a response. - the bottom blue background reply link replies to the user who created the thread and
notifies them that there is a response.
i sometimes find responses that are meant for me but since the blue link was used i dont get a notification and see the response only by chance often after days. 
i am sure this happens with other users as well. so when you are using one of these reply links please use the appropriate one depending on who you want notified or not notified.
thanks.