Here are some things you can check:

• In Groups.io:
  • Are the missing messages listed on the Recent Bounces page for your Groups.io account?
  • Are the missing messages listed on your Email Delivery History page in that group?
    See Checking your email delivery history for a group.
  • Were the messages posted to a topic that you muted?
  • Is your Groups.io account in Bouncing (or Bounced) status?
    See Dealing with bouncing accounts.
  • Is your email delivery preference for the group set to Individual Messages?
    See Email delivery settings.
  • Is your message selection preference for the group set to All Messages?
    See the Message Selection topic in Advanced subscription preferences.
  • Do the missing messages contain a hashtag that has the No Email property?
  • Do the missing messages contain a hashtag that you have muted?
• In your email client or the email provider’s webmail interface:
  • Have you set a filter that blocks or redirects some (or all) group messages?
  • Are group messages getting delivered to your email service’s spam (or trash or other junk mail) folder?
  • Are you using Microsoft Exchange for your email? If so, send an email to support@groups.io and inform them that you are using Exchange. The support team will make an adjustment so all group email messages sent to you will have the From line rewritten, which will fix the issue.