Since last week I don't receive anymore email notifications when someone replies to thread i am subscribed to.
This worked for me today.
You might not receive emails for any of the following reasons:
You did not visit the thread after the last email was sent (Each email notes: "There may also be other replies, but you will not receive any more notifications until you visit the forum again.")
Your notifications might be set to something besides "Instant notification by email" (Even then, there is something like a half hour delay)
Your email provider might be sending the messages to Spam or Junk Mail