It might happen that your last guest left a general positive review but it doesn't reflect the general score of their review. Please find the reason for this below:
Once the Booking.com guest checks out, Booking.com sends an email to ask them to review their stay. Guests have 90 days to do this.
Guests are being asked to select an overall score from 1 to 10 to rate their experience at your property. This is the only mandatory question to submit a review.They can rate other services, such as cleanliness, comfort, value for money, facilities, location, and staff. These ratings are optional.
The overall score is neither an average nor a sum of the 6 subcategories. They are completely independent from each other. The six categories are selected by the platform and may not make up for the full experience your guests had at your property. For some guests, non-existing categories such as general design, family-friendliness and other criteria may be important and this would be reflected in their overall score. For more information, please visit Booking.com’s website: Everything you need to know about Guest Review Scores
If your guest left a bad review, you can read more on that topic here: Can I delete a review left by a guest? My guest left a bad review, what can I do?