Person and Group - SharePoint Groups - does not show all members of group

[Lee-Martin]

I've a SharePoint Person and Group column in a SharePoint List. There are about 400 people in the group.

I search for the name David in my List. I get one result. However there are 7 Davids in the group. 

Does anyone know why this is occurring, and how I can get the contents of my group to appear correctly in the List column?

[#2]

Can anybody help?

We've still not resolved this, Graham's are getting 6 results when there are many more, and a Jame (with 4 characters) doesn't get any results even though there is only one James.

This is becoming an urgent issue.