Server Side Popup Window not getting Displayed ASP.Net

Works fine for me

 protected void Button1_Click1(object sender, EventArgs e)
    {
        ClientScript.RegisterStartupScript(this.Page.GetType(), "", "window.open('Print.aspx','Print','scrollbars=yes,height=400,width=900');", true);
    }

It is difficult to say what can be issue

1. There can be some javascript error on page

2. Try on some other brand new page

1. Create a new test page and try that as there is nothing in the script which is causing problem