Hello,
Go to partner record of those followers and see whether following 2 fields are not wrongly set up ?
1. Opt-Out (By tick this, system will prevent to send any kind of notification to customers )
2. Receive Inbox Notifications by Email - Never ( By select this, customer will not receive email, they will only see activities in their Mail Wall )
Even if by properly set up above 2 options, if your customers not receive emails then do following things.
1. Show the log over here. So I can understand the error.
2. Which outgoing smtp server you are using. ( Gmail, yahoo or anyone else ? )
3. Go settings >> Technical >> Email >> Messages and open the message which is related to particular sale order. Then Please varify the followers are in "Notified Partners". If not then you have to paste your log over here.
4. Go Settings >> Technical >> Email >> Emails & remove filter "Outgoing". Please look whether your email records are in state of "error", which may prevents emails to send out from system.
I am eager to solve this problem. All the very best,