Combobox databinding with datagrid

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C#

Hi everyBody!
i'm building an application in wpf and I'm facing a problem when binding my combobox to my datagrid. I've a combobox cbTeacher on my form and a datagrid dgteacher.


Firstly I fill my combobox using a service:

exemple: cbTeacger.ItemsSource = new ClientServices.ClientService().Services.svc_load_Teacher();

because the teacher informations comes from an other Table.
so when i drop down my combobox I have the entire list of my teachers.

Secondly
The combobox is binded to my datagrid (dgteacher) so that after completing the form and saving data to database I can see all lines of informations stored in database directly from the datagrid.

The problem is , when navigating through each line of my datagrid I don't see the informations bound to my combobox.
Clearly the combobox stay like there is no selecteditem or selectedindex. I don't know why??

Someone can help ?