Do NOT send a message to your buyers after you deliver an order either
a: asking for a review or b: asking when or why they didn’t review.
This will get you in hot water if the system flags the message OR if the buyer is annoyed that you are messaging them and your messages can be marked as spam which can get your account a warning or worse.
The buyers are already prompted upon delivery to rate the service they received, and I think they may even be reminded by Fiverr as well before the order closes out.
You should do NOTHING. Sit on your hands and do not bother the buyer to give you a review. Unless you want your freelancing career here to be short-lived.
I have never received a message on either Etsy or Ebay after I have bought something from the sellers to review my purchase - and would be annoyed if I did. I know how to review if I choose to do so. So, I wonder if those two platforms also have this type of rule where they frown upon you bothering clients to leave feedback.