This has happened to me off an on for quite some time. I'll all of a sudden stop getting the e-mails and then a few weeks later they start back up again. I see no rhyme nor reason for it and it's not from me changing any settings on my end. I don't sweat it b/c/ I just log in and pull up all my subscribed threads and everything is up to date there. So the subscriptions don't turn off. Just the notifications. Until they turn themselves back on.