Group members from Contact do not appear in Mail address 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.