It is worth noting that if the CRON task that sends out the digest emails is not functioning then nobody signed up for the digests will receive them. I had the initial impression that this is not the case. But having said that…
WordPress has always had a, shall we say, slightly difficult time with CRON tasks. For some reasons they do sometimes – although rarely – turn themselves off.
Simple:Press actually gives you a admin page in the forum toolbox section that allows you to monitor and see all of the CRON tasks that are set up. This includes when the next task is scheduled to be actioned. If you keep a watch in those then they should peel off when done and then get added back into their position for next time.
If the more general WP tasks are working but SP tasks are not then the easiest way to get them re-scheduled is literally to deactivate the SP plugin (NOT Uninstall! – just deactivate) and them immediately re-activate it again.