You could check the email logs for more clues? Is there anything under skipped? Filtering by ‘digest’ for sent and skipped may help narrow it down. If you can pin down if they’re being generated at all that may help find the cause.
If you still have the default default email digest frequency as a week, and the default suppress digest email after days as 180 days, then a summary email should be generated for anyone who hasn’t seen the forum in between those times.
Here’s the FAQ, if it helps?