There can be reasons why a buyer won't leave feedback. They are busy, away from home, or just plain forgot. If they chose not to leave any, best not to ask in case that makes them grumpy and they leave something not nice in response. No feedback can be better than irritated feedback 🙂 just think of the sales, the buyers were happy enough with everything not to want to complain, and you will get there soon. Feedback is voluntary.