About 5-6 weeks ago Joomlart did some maintenance on the forum and it was down for a few hours. When it came back ‘Instant notification by email’ was turned off by default for new thread subscriptions. You would still get emails for new posts on threads you were already subscribed to before the maintenance but not after.
Go to ‘User CP>Edit Options>Scroll down to ‘Default Thread Subscription Mode‘ and make sure ‘Instant Email notification‘ is enabled to turn it back on.
Then go to ‘User CP>List Subscriptions’ and you will see all the threads you have posted in and you can tick which ones you want notification for or not.