Why is html node collection null even though the site is fine?

Question

Parsing

Like on the site everything is fine, but through the code everything is bad. Either

website :

private HtmlNode CheckXpath(string xpath)
        {
            if(!string.IsNullOrEmpty(xpath))
            {
                if (html.DocumentNode.SelectNodes(xpath) != null)
                {
                    return html.DocumentNode.SelectNodes(xpath)[0];
                }
            }

            return null;
        }

xpath = //*[@id='gr_case_partps']/table/tbody/tr/td[2]/div/ul/li/span/a