Unable to add some conacts to groups on Windows Phone 8.1

[jwerner74]

I'm running Windows Phone 8.1 on a Nokia Lumia 928. I've been using contact groups to manage emails and texts to the parents for my daughter's sports teams. Up until now, I've never had a problem. But now, when I try to add certain contacts to a group (an existing group, or a newly created group), the contact will show up in the group list for a moment, then disappear. It seems to only be a problem for contacts that have been created in the last few months. If I try to add a contact that has been part of my list for some time to a group, the contact will stay, as intended. I'm really clueless as to what the problem might be. The phone numbers and email addresses of these contacts are properly formatted, as I have successfully sent them all emails and texts in the past.

Thank you in advance for any help you might be able to give.