It is entirely possible that Apple has automated systems in place to weed out non organic reviews. For instance, ratings and reviews gathered from incentivized sites my show up initially in the app store but may also disappear suddenly. There is surely some clean up job that Apple runs in the background for this.
While I am not implying that you have used any inorganic methods to get reviews for your app, organic reviews are much better, more effective and definitely long lasting than any other methods you may employ. I have written a detailed post on this on my blog:
Why Are App Ratings And Reviews Important For Your Overall App Store Optimization Strategy?
How do you get ratings and reviews in the app store?
Another way to confirm if something is actually wrong is to capture events in your app using Google Analytics or other solutions. You can monitor the number of hits on the OK button for ratings pop-up and the percentage of those that usually write a review. So if you see no reviews in the app store but GA tells you that the number of hits on your OK button is still high, then you may have something to worry about. On the other hand, if the hits on the OK button is missing then you probably missed something within your app.
