Hi Sergio & Nicole,
We have a similar issue...sometimes not getting enough reviews to meet the 50% Superhost requirement.
We noticed, some local guests do not write reviews in order not to get one from the host.
Not because they were unhappy, but for privacy reasons, not to leave a trail of their travels.
(If they were good guests, we respect that, if not, we write one, to warn other hosts.)
But some guests are just a bit lazy or not familiar with writing reviews (...despite the Airbnb reminders). So, after a few days waiting, you could let the guest know, you would be happy to leave them a review. Tell them, having reviews is a good thing, as many hosts only accept reviewed guests. You'll find, most guests respond, by writing a review after this gentle reminder.