Sometimes an error:UnityException: WWW is not ready downloading yet，Because so many data transfer？

public IEnumerator LoadTrackHeader(string getsoapName) { string ipaddress = "http://192.168.88.1:8080/SatDataum.asmx/TdTrackHeader"; www_theader = new WWW(ipaddress); yield return www_theader; TrackHeader_str = www_theader.text; if (!string.IsNullOrEmpty(TrackHeader_str)) { json_trackheader = JsonMapper.ToObject (TrackHeader_str); foreach (var item in json_trackheader) { StartCoroutine(TdTrackdata("TdTrackdataCount", item["Track_Name"].ToString(), item["Tid"].ToString(), 0, 200)); } } }

public IEnumerator TdTrackdata(string getsoapName, string trackName, string tid, int startindex, int count) { List listdata = new List(); string ipaddress = "http://192.168.88.1:8080/SatDataum.asmx/TdTrackdataCount"; www_thdata = new WWW(string.Format(ipaddress + "?tid={0}☆tNum={1}&count={2}", tid, startindex, count));

         yield return www_thdata;
         if (string.IsNullOrEmpty(www_thdata.text))
         {
             if (!string.IsNullOrEmpty(Trackdata_str))
             {
                 json_trackdata = JsonMapper.ToObject<JsonData[]>(Trackdata_str);
                 foreach (var item in json_trackdata)
                 {
                     TdTrackDatumEntity tddatum = new TdTrackDatumEntity();
                     tddatum.Dtid = item["DTid"].ToString();
                     listdata.Add(tddatum);
                 }
                 dir_trackdata.Add(trackName, listdata);
             }
         }
     }

my platform is webplayer.