iMac using OS X 10.8.5. When using a Group from Contacts in a Mail address, all the Group list does not appear in the Mail address and those missing do not receive the email.
Try creating a duplicate of the group and trying again to see if it the duplicated group works. Also, check that all the contacts have a valid email address on their contact card as this could be what is causing issues with the Mail App.
I had this problem too. I removed the existing contact card from the group and added it back. That worked for me.
I had this problem, too. Making a copy of the group didn't help. I fixed the issue by making a new contact card for that person (from scratch), deleting the old card, then adding the new card to the group.