@Id0.......The problem is many guests or hosts wanted the review window extended because circumstances did not fit their particular situation.
They wanted the review window re-opened or extended for... a day or two.....a week...sometimes even longer and the whole review system became unworkable.
In the end Airbnb said 14 days is it....reminders are sent out in the 14 day window but if a guest or host does not write their review in that time, that is the finish of it....absolutely no extensions will be offered!
I am sorry @Id0 but that is the way it is.
What you can do though is offer the guest the opportunity to leave you a reference! It is not as good as a review but it will appear on your profile page directly under your reviews.
Here is the link......
https://www.airbnb.ca/users/references
I am sorry that your guest missed the review window but I hope this offers you and your guest an alternative.
Cheers......Rob