I have a customer who told me that is a little hard to find the way for giving a rating to a product. You should make it easy to rate a product. Also maybe sending them an email asking them to do it several days after the purchase, as well as enabling the complete written review system.
The text review system is on the roadmap: Epic Developer Community | Fab Roadmap
But it not gonna come any time soon seeing how slow the fab team is delivering a single task from the roadmap.