That is most likely a temporary issue where the template(s) for digests didn't exist at the time of sending. If it happens again, feel free to let me know
When I run it, I get the test digest which shows up fine in my email, but my members keep telling me there is no content in the digest they are receiving.
Well, the test uses the exact same function to send the digest as the cron job does, so I'm very much unsure.
If all else fails, you can always deactivate the Digest feature in vBMail and revert to using the Newsletter mod - the functionality, at the time of writing, is pretty much identical.