Ask for good reviews

Only 80-percent of my Booking,com guests will post their online reviews after they leave.

I also have a guestbook in my rental suite for guests to leave their written comments.

Very politely you can ask them if they are satisfied with your service and if not what changes you should do...to provide better service and that leaving you a review is very important for you and for other guests interested in your hotel.

You can mention to them that if they had a nice stay to tell their friends about their experience.

Usually after my guests leave I send them a thank you message reminding them of leaving me a review.

Best wishes.

Dear Nairoukh, I believe personal approach is key. You, or your staff, must somehow stay in touch with your guests throughout their stay at your place.  We are in hospitality, we must transform our inner selves into someone that connects to and serves the guest. Don't be afraid to ask your guests about it. This is 2020, they also understand that 'good/positive review = key to success'. Good luck with your reviews.

Marcus, I agree with you all the way... guests feel appreciated when we try to communicate with them as some are shy to start a conversation or express their needs.

Have a good day.

Also, when contacting them directly, ideally just before they leave or not long after, ask them  something like that you hoped they enjoyed their stay and if there was something they felt you could improve upon that they contact you personally. Explain how their feedback and suggestions will assist in improving your ongoing service. By asking this it shows how committed you are to making a guests stay better and hopefully  if there was anything they weren’t happy about that they have addressed it with you and not included it on their review on booking.com’s website for everyone to see.