Hi Chuck, I know this is a five month old thread but I'm still stuck with this.
As far as I can tell this is only affecting a small number of members as I mentioned before.
This is what happens:
User uploads a picture but does not automatically get notification by email that a comment has been made.
User comments on someone else's picture and they don't get notification by email that they have a reply.
User subscribes to a category and only gets notification of recent uploads but not about comments.
The only way to get comments notification for these users is to open an individual picture and choose from the photo options receive email updates and then the user will get comments by email.
The vast majority of members don't have a problem... only a select few.
I've had the domain investigated and have since moved them to a different domain.
Any ideas at all please.