Buyer Rating

Just wondering what factors make the buyers to rate an item or not. I see some files gets 5-6 ratings within 15 sales and some takes around 30-40 sales even for 3 ratings.

Mysteriously one of my item didn’t get even single rating for the recent 100 sales! (7 ratings at around 75th sale and still shows 7 after 175th sale) Is there any glitch in the system or the way ratings exposed to buyers?

Just a suggestion: In the downloads page, above the stars, adding a message something like “Rate this item:” should make it more understandable. After making rating, it can say “You rated:”