I've been experiencing the same problem with the individual notifications.
It reminds me of Facebook. Only a handful of my friends on FB actually see my posts. I'll comment on theirs and they're like, "Huh? You're still on Facebook? I thought you stopped months ago?"...and yet I've been posting every other day. Even my own wife doesn't see my posts on her feed anymore.
I guess FB did their own "Panda" type update or something. Maybe the only people who see my FB posts are the ones who always "Like" and comment on my posts, and FB just stopped notifying the rest.
Maybe HubPages is doing it the same way? I still see hub notifications for the handful of hubbers that always comment on my hubs (and whose hubs I always comment on), but the others have just disappeared off the radar.