If some digests are getting through but others aren't, compare the number of members you have with the number of daily hits on the particular WSN installation you're dealing with here and the number of emails per queue being sent. Go to Admin Panel -> E-Mails -> E-mail Preferences -> "Number of emails to send at a time from queue:" to check how many are sent per page. If that number is 10 and you have 10,000 members integrated from a discussion forum, but the links directory you're dealing with only gets 10 visits a day, only about 100 digests will get sent out per day. This is because PHP can only run when someone requests a page, so it can only send out digests when someone visits. You can increase the number sent per page, but not too much or the pages will become noticably slow or error out at very large numbers.
A solution to get more emails sent without slowing pages is manufacture more (fake) visitors. You can do this on a unix or linux operating system by setting up a system cron -- please note this is NOT the same as the crons page in the WSN admin panel (WSN crons are php-based so can only run when people are viewing pages). Your hosting control panel should offer you a way to set a cron. If you have CPanel, you'll find a "cron jobs" option in the menu. Select "standard" experience level. Supply