C# screen scraping an ASP.NET web forms page - POST request not completely working
If I understand you correctly, then selecting an item in the dropdown will cause a POST to be performed, and the server alters the available options in another part of the form. The server will then include the current value of the dropdown in the __VIEWSTATE field value.
When you perform the scraping, you should make sure that the __VIEWSTATE contains the desired value for the dropdown. To investigate further, try to decode the viewstate from the server and see which values are sent back.