Have advertisements been added to the notifications?
If that's the case, then if notifications have been sent to me, that might be the cause of my not having received them -- and I'll never see them ever again.
If you want an email for EVERY update on your subscriptions set to Immediate, make the change to send a notification every time. This is defaulted to off, because it can wind up being a lot of email. The default will send you one update, and no more until you're active on the site again.