There are two things you must check to make sure that the digests will work:
Be certain that bbPress Notify (No-Spam) – the free plugin – is working correctly and that your test user gets notifications. The digest add-on intercepts the notifications sent by bbpnns and saves them in the queue.
Check the test user’s Digest preferences. Make sure it’s set to one of daily, weekly, or monthly (daily is best when testing). Optionally, you can enable Instant Notifications as well.
You can also try the Dry Run tool in the bbpnns support tab and see if the Digest add-on registers any entries.