Welcome to the FlyerTalk. I am to lived as a resident in Japan. As for why the postman did not leave the notice? That will be anybody's guess. Even in Japan mistakes do happen, possible that the postman forgot to leave the notice.

As for the second delivery attempt, I thought (not 100% sure) that Japan Postal Service has discontinued automatic second delivery attempt few years ago. After receiving missed delivery attempt notice which is 「ご不在連絡票」you can call the post office to arrange the second delivery attempt. I think you can also arrange second delivery attempt via website but I am not 100% sure. But the post office may not automatically attempt second delivery.

If you do not call in for second delivery request then on the missed deliver attempt notice there is 「保管期間」which indicates the date. Post office will hold your item till this date. If you do not call in for second delivery attempt or come to the post office for pick up by this date, then the post office will send the item back to the sender.

As to why you did not receive the missed delivery attempt notice, I do not think anybody here can answer that. Good luck.