Combo Box Row Source Problem

I have a form F_EXTRACT whose record source is a table named T_EXTRACT.
The form contains 2 controls, a Text Box named Surname and a Combo Box named Member_ID.
Column 1 of the combo box is bound to Member_ID (in the T_EXTRACT table).
Column Count is set to 3.
In Row Source, I have the following SQL statement:-

SELECT [T_MEMBER].[Member_ID], [T_MEMBER].[Surname], [T_MEMBER].[Initials]
FROM T_MEMBER
WHERE ((([T_MEMBER].[Surname])=[Forms]![F_EXTRACT]![Surname]))
ORDER BY [T_MEMBER].[Surname], [T_MEMBER].[Initials];

I wish to populate T_EXTRACT with selected Member_IDs but as the MEMBER_ID table contains many hundreds of records need to reduce the search through the display in the combo box.

The above method (by entering a surname in the Surname text box) works fine for the first entered surname but second and subsequent surname entries result in the details for the first entered surname being displayed in the combo box.

I've tried binding Surname to a field in the F_EXTRACT table but this makes no difference.

Any ideas would be much appreciated.