Problems with ScriptManager.RegisterClientScriptBlock

My function btnDownloadArchives_Click well summarized.

protected void btnDownloadArquivos_Click(object sender, EventArgs e)
{
            Response.ContentType = "application/octet-stream";
            Response.AddHeader("Content-Disposition", "attachment; filename= " + nomeArquivo);
            Response.CacheControl = "Private";

           //Aqui dentro tem uma outra função qualquer.


            Response.Flush();
            Response.Clear();
            Response.ClearContent();
            HttpContext.Current.ApplicationInstance.CompleteRequest();

            EscreveErroNaTela();

}           

Inside the WriteErnaNaTel () function has the following code.

ScriptManager.RegisterClientScriptBlock(this, GetType(), "Alerta", "alert('Alguns arquivos não puderam ser baixados. Clique no botão erros para maiores informações');", true);

This code ScriptManager.RegisterClientScriptBlock works fine, the problem is that in the function btnDownload_ClickArchives has some conditions, and when entering this condition it fills some Response properties, after a Flush, Clear, Clear content.

When it fills these properties my ScriptManager does not work, no alert is displayed, what may be happening?