Groups email lists occasionally don't send some members emails to some other members

A few months ago one of the google.groups.com email lists I have used for years with my gmail email address has started to not deliver some messages to me and other users. I can't figure out any pattern to which email goes undelivered to me or other users. A message will be delivered to some users but not others. Often I will discover the missing email because someone else replies to the email but I did not get the original email that is being replied to. Sometimes the emails will come through hours later. If I login to the web interface the emails are all there. This was occurring on a group which was administered by google.groups.com but the email address for the group is not at googlegroups.com. I suspected this domain issue was part of the problem since another email group with a googlegroups.com domain was not having this occur--but just now I noticed it happening on the other group. I have not changed any settings and have it set to email me "Each email". Emails are not going to spam folder. Any idea what might be happening? Do I need to switch to another listserv service?