Service of chamber summons to ex-parte defendant to amend plaint

Sir / Madam, Our civil suit is proceeded exparte against some defendant & some defendant contesting suit. We have taken out Chamber summons to amend the plaint & it has been sent to all Defendants. Said summons have been duly served to those defendants who are contesting suit but defendant against whom suit is proceeded ex parte are found to be not available at given address. So in this case is it not sufficient to serve chamber summons to those who are contesting suit? Is it possible to proceed for amendment without serving to those who are proceeded ex parte? Thank You.