This is a very common issue and following are the cases which are reported to this :
1. BuddyPress system : Activation email not being sent to users on registration
2. BuddyPress system :Notification/Messages emails not being sent to users
3. WPLMS System : Vibe Short-code Form submission emails not being sent to users
Steps to debug :
1. Check if WordPress is able to send out emails.
- Go to WP Admin -> Users -> Add New
- Create a new user and enter a valid email
- Check on send password to user
- Verify if the email is recieved
2. Check if BuddyPress system is working properly:
- Check the BuddyPress FAQ on email ( BuddyPress isn’t sending out emails (eg. activation emails, email notifications) )
- Check with your web host in case they’re blocking out emails.
- Check if registration and activation pages are connected in buddypress. WP Admin -> Settings-> BuddyPress -> Pages -> Activation
3. Check for javascript errors (important for step 3)
- Check if there are any javascript errors reported on page. refer link
- Also try re-saving permalinks to %postname%