why buyers don`t give any positive review after giving most excellent service in time?

Hi zooel,

This is very common and can be for many different reasons.

Some buyers simply don't care as they gain nothing from leaving feedback, some don't even realize it's a thing and i'm sure there would be hundreds of more reasons. 

All you can do is provide the best possible Service and leave a nice, not pushy reminder in your Delivery.

Regards,
Karoshio

Will I think it will be hassle for them, they maybe don't feel doing it. But for the one doing the service just don't expect that everyone will be giving a feedback if you provide them a nice service. Most customer don't but still even if they did not provide a feedback it doesn't mean that they don't appreciate the your service. Just continue doing a quality service and you will still be the one to benefit at the end.