Keep in mind that there are two parts to a drop-down list box: the stored value and the displayed value. The displayed value is pointless to this discussion -- it simply is the text shown to the user so that they don't ever have to see the stored value.
The way the drop-down list box is supposed to work is this:
If a stored value matches the actual value in the field, then the display value is shown to the user.
If there is no stored value which matches the actual value in the field, then the actual value in the field is displayed in the drop-down in place of a display value.
If you are getting "Select..." then it would seem that your actaul value in the field has been reset to blank.