Have you ch checked a few of the accounts not receiving to see if they have categories &/or tags muted? I believe if you have it muted th digest will not include topics.
Otherwise the team is likely ty best resource.
Another the ng with your mail server did you add dkim DNS record to your domain? Not sure if this is needed from all SMTP providers. But can help with emails not marked as spam and ir blocked. Have you asked users to check spam?