XSLT Transformation From DataSet

Hi, 
 
I have some problem with XSLT Transformation. I don't know why. When I'm using XMLDataReader directly. It can work. But If I save all the data first in DataSet. Then use DataSet to use XSLT Transformation. It doesn't work. Strange. Although I have checked that the XML they both produced was the same. 
 Thanks in advance. 
 Here are the codes : 
 Work properly

 //Retrieve the connection string from the web.config file
            string connString = System.Configuration.ConfigurationSettings.AppSettings["connectionString"];
            SqlConnection sqlConn = new SqlConnection(connString);
            //Open the connection to the database
            sqlConn.Open(); ...