Combo Box Selection is Blank

I have created a form which includes the fields [Unit], [Rate] and
[ExtendedCost] which is the sum of [Unit]*[Rate]. The [Rate] field is
actually a combo box which allows the user to select a rate from a query.
When I click the arrow for the combo box to expand, all the rates are listed,
however, when i make a selection the combo box remains blank. However the
[ExtendedCost] field still calculates. The table from which this form was
created works perfectly and the combo box functions and displays the
selection. Can anyone help?

Correction to my earlier post. The combo box consists of 3 columns -
description, rate, unit of measure. Although the bound column in Column 2,
it is now bringing in the description instead of the rate. Again the
[ExtendedCost] field is correct.

"Reva" wrote:

I have created a form which includes the fields [Unit], [Rate] and
[ExtendedCost] which is the sum of [Unit]*[Rate]. The [Rate] field is
actually a combo box which allows the user to select a rate from a query.
When I click the arrow for the combo box to expand, all the rates are listed,
however, when i make a selection the combo box remains blank. However the
[ExtendedCost] field still calculates. The table from which this form was
created works perfectly and the combo box functions and displays the
selection. Can anyone help?