I feel like game ratings aren't promoted quite enough. As an anecdote, I have around 175k downloads on one my games, and 50 ratings.
On the web version there's the tiniest button in the world on the side of the page (see below if you too have missed it.) And in the app it seems like there's no way of rating, or they're even more hidden.
My suggestion would be moving the ratings to a more obvious place. If someone is going to comment, chances are they're willing to rate while at it.