If he was wounded in battle and you disengaged to replenish your troops, he wasn´t fit to participate in the new battle. Those lords (too little health left to spawn at the beginning of a new battle) are not capturable.
Leave him be till he recovers some more health, engage again and recapture him. You can press any unique spawn for a gem as often as you capture him/it.
Leave him be till he recovers some more health, engage again and recapture him. You can press any unique spawn for a gem as often as you capture him/it.