insert a row problem

I use a web service to add a new record to the DB.
it works :)
but not always :(

The following code works for one table of mine and not for the second (all the feilds are simple text without no roles on ms-access DB).

Why?

PHP Code:

#region add - adding a new course model to the DB. /// <summary> /// This service is used to add a new model to the DB. /// It returns its caller a bolleanic answer whether or not /// the operation succeeded. /// </summary> [System.Web.Services.WebMethod(Description = "This service adds a new model to the data base.")] public bool add(String code,String name,String hours,String totStud) { bool added; // a variable that indicated whether the new row was added OleDbConnection conn = new OleDbConnection(root); conn.Open(); String command = "select * from model"; OleDbDataAdapter adapter = new OleDbDataAdapter( command,conn); DataSet ds = new DataSet(); OleDbCommandBuilder cm = new OleDbCommandBuilder(adapter); adapter.Fill(ds); DataRow newrow = ds.Tables[0].NewRow(); newrow["modelID"] = code; newrow["name"] = name; newrow["hours"] = hours; newrow["maxNumStud"] = totStud; ds.Tables[0].Rows.Add(newrow); adapter.UpdateCommand = cm.GetUpdateCommand(); try { adapter.Update(ds.Tables[0]);// ( ds,"students"); added = true; } catch { added = false; // if the adapter fails to update the DB } conn.Close(); return(added); } // add method #endregion