This user has 3 PCs involved in this issue. They are each at a different site.
PC 'A' is running Outlook 2010.
PC's 'B and 'C' are running Outlook 2013.
The issue is with one of the contact groups the user has created. There are 5 people in this group.
The expected behaviour is that once the group is created, it will be visible on all 3 PCs. This does work.
The issue is that, sometimes, when the user is on PC 'B' or PC 'C', the contact group will change. It will be missing people, usually only one will be left. This never happens on PC 'A'.
I took a look at the details of each contact and noticed one had an incorrect email, I fixed that hoping it would stop the issue.
It has occurred once again since that change, so it didn't help.
My first thought is that it has something to do with the different versions of Outlook installed but am not sure what to do or check based on that.