Starting a script from vb code won't work anywhere in imageButton_Click method

I am trying to call a script to uncheck all checked checkboxes in my gridview upon a button click. The button is an image button that also downloads images that correspond to the rows that are checked. This line of code works in other methods fine but for some reason I cannot figure out, won't work anywhere in this sub.

Protected Sub dwnld_Click(sender As Object, e As ImageClickEventArgs)
    Dim found As Boolean = False

    Using zip As New ZipFile()
        zip.AlternateEncodingUsage = ZipOption.AsNecessary
        zip.AddDirectoryByName("Drawings")
        For Each row As GridViewRow In gv1.Rows
            Dim c As CheckBox = TryCast(row.FindControl("cBox"), CheckBox)
            If c.Checked Then
                found = True
                Dim filepath = serv & row.Cells(1).Text & "B" & row.Cells(3).Text & ".TIF"
                zip.AddFile(filepath, "Drawings")
                'TryCast(row.FindControl("cBox"), CheckBox).Checked = False
            End If
        Next
        If found Then
            Response.Clear()
            Response.BufferOutput = False
            Dim zipname As String = [String].Format("Drawings_{0}.zip", DateTime.Now.ToString("yyyy-MM-dd-HH:mm:ss"))
            Response.ContentType = "application/zip"
            Response.AddHeader("content-disposition", "attachment; filename=" + zipname)
            zip.Save(Response.OutputStream)
            Response.End()
        Else
            If gv1.Rows.Count > 0 Then
                reSetup()
            Else
                initReSetup()
            End If
        End If
    End Using
End Sub

    ScriptManager.RegisterStartupScript(Page, GetType(Page), "Click", "clickb()", True)