thread subscription not working...

My members tell me they are not always getting email's to them when a thread the have subsrcibed to has been replied to. I have been told they may get one or two emails, but a lot goes unemailed.

I have done some searching and found that someone suggested looking at their functions.php and newreply.php files...my functions.php from:

// ############## Send out email notification ############

to

// ###################### Start doshutdown #######################

Are the exact same as the unmodded files included in the vBulletin zip file.

I do believe everyone is getting their confirmation emails, though if i recall one or two said they didnt.

Any clues to what may be going on?