A few things to check for those users who are getting notifications:
- Do they have Digests enabled in their profile screens?
- Do you have WP Cron enabled or a real cron job?
- Is there any data in your bbpnns_digest_queue table?
If you haven’t yet, please have a look at https://usestrict.net/bbpress-notify-no-spam-digests-documentation/ for an in-depth understanding of how the Digests add-on works, along with some troubleshooting tips (found under the Testing Mailouts section).
Let me know if you’re still having trouble after this.