LDAP Authentication fails on web server

Hi,
I have used the below code for LDAP authentication which works perfectly on my machine, but fails to authenticate on web server. I have tried the code locally on web server and it works. The only problem is it fails to authenticate on web server.
        using (DirectoryEntry entry = new DirectoryEntry("PATH", null, null, AuthenticationTypes.None))
            {
                DirectorySearcher search = new DirectorySearcher(entry);
                search.SearchScope = SearchScope.Subtree;
                search.Filter = "(SAMAccountName=" + user + ")";
        ...