Workflow Assignment to AD user

Using GP2015R2 Workflows.

On the server we can click the user assignment and in the next window browse the AD to find the required user.
On the PCs we are unable to do so - the Search Box just returns searching.  Previously assigned users do not appear in the Assigned to Box.  On the PC we are able to access and search the AD in non GP widows (e.g. User Security).  We are also able to access all AD related shares and resources.

How does GP communicate with the AD in this search?

Thanks