Hi James,
A few things to help troubleshoot:
1. Can you provide a screenshot of your Settings > Forums > Email
Notifications section?
2. Do you get the emails immediately when the Digests plugin is turned
off?
3. Can you check for the existence of rows in the bbpnns_digest_queue
table using phpMySQL or some other MySQL client such as the SQL Executioner
plugin?
Cheers,
Vinny