MySqlDataAdapter return null value in wcf web service

Question

public static DataTable SelectMethod(string _select)
        {
            DataTable _dtsel = new DataTable();
            MySqlDataAdapter _mysel = new MySqlDataAdapter(_select, con);
            // _mysel.SelectCommand.CommandTimeout = 900;
            try
            {
                _mysel.Fill(_dtsel);
            }
            catch (Exception ex) { string a = ex.Message; return null; }
            finally
            {
                _mysel.Dispose();
                ConClose();
            }
            return _dtsel;

        }

 

This code working fine
but sometimes when we try to call this service repeatedly ....sqldataadapter returns null value... pl help me what is the probelm ... /