Dropdownlist not responding to first and last item

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VB

ASP.NET

I have an application in asp.net/vb.net and sql server express 2008. On the form i have two dropdownlists one for selection of years(from 2006-2013), the other is selection of grades(from 1-8). The other controls are listbox and a button. I want the results to be displayed in listbox upon selecting year and grade from dropdownlists and on click of a button. The select query works great in sql server. But in the application it is doing what i want except for year 2006 and 2013 but data for these years is there. Iam using dropdown selected value. Why the query is not selecting records for the first and last item in a dropdownlist.

the code below

Protected Sub Button2_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button2.Click

        Dim queryString As String = _
                "SELECT DISTINCT Code FROM enrolnment WHERE grade ='" & DropDownList2.Selectedvalue & "' AND Year ='" & DropDownList3.Selectedvalue & "'AND iActive = '1';"
        Using connection As New SqlConnection(connectionString)
            Dim command As New SqlCommand(queryString, connection)
            connection.Open()
            Dim reader As SqlDataReader = command.ExecuteReader()
            Dim i As Integer = 0
            command.CommandTimeout = 300

            While reader.Read()
                'i < Listbox1.Items.Count
                ListBox1.Items.Add(reader.GetValue(i).ToString())
                command.CommandTimeout = 300
            End While
            reader.Close()

            'End Try
        End Using

    End Sub