Maybe something happened to your set option for notification?
On your User CP page of Subscribed Threads what does it say under "Notifications"? This snip shows two options that are on my page.
You can check what your "Notification" is set for the subscribed threads by going to your "User CP" and scroll down to "Edit Options"
Then scroll down the center of the page until you see "Messaging & Notification". There will be a box for "Default Thread Subscription Mode"
The drop down box will let you choose:
Do not subscribe
No e-mail notification
Instant e-mail notification
Daily e-mail notification
Weekly e-mail notification