Reply To: some users get email notifications, others don’t
Emails are notoriously difficult to fault find, as so much is beyond the website.
So start with
1. You should be aware that many spam filters strip messages that do not come from the correct address. So if your site is mysite.com and your email address in wordpress setting/general is fred @gmail.com then it is likely that messages will be dumped in transit. You need to set up email to come from your site eg fred @mysite.com, your hosting provider can help if needed.
2. Different email providers have different rules, so an email to fred @aol.com may be treated differently than fred @google.com if google and aol have different rules as the email passes through their servers, so some may get through and others not.
3. Just bbpress?
Then you need to see if this is wordpress wide or just bbpress.
Try https://wordpress.org/plugins/check-email/
4. Some people have fixed it with this plugin https://wordpress.org/plugins/asyncronous-bbpress-subscriptions/
