Have you remembered to set up a cron job? I hadn't!
Do a search for "cron" in the forums--basically you have to automatically open the page your.site/admin/cron.php for the system to do all the little jobs it's supposed to on a regular basis, like posting out digests etc. You can either have a machine you own do it (if it's on all the time), or get someone else's host to do it for you...
A word of warning: most of your users will probably be subscribed to the forums without having set their email option to recieve digests. If there's been a fair bit of activity in the meantime, they may receive many emails suddenly all at once when you switch on cron. A well worded apology and explanation in the News forum should placate them!