guest reviews

Today, I received notification that a guest from November had left a review. While he provided a 9.0 review overall, he left 7.5s in many categories.  I wanted to ask him what my team could do better. When I tried to message him through Booking.com, I received error messages every time. Are we not allowed to message guests after the fact? Or, more likely, did this person leave me a bad review and then quit the platform?  It is crazy that we, as hosts, have no means to respond to prior guests after they have provided a review.  Any guidance from the community would be most welcome.

Thanks in advance.