We suggest you below steps one after other after you understood that item is received by buyer:
day 1: a mail to buyer asking him about the item he received and also include the directions to leave feedback/acknowledge item receipt and your contact details.
day 2: resend the same mail.
day 3 morning: SMS them.
Day 3 evening: call them and ask them to let you know when they would be online so that you can call them again at that time to guide them on feedback.
Hope this helps and you will surely receive all feedbacks.
Regards
