Only the reviews selected by Udemy appear in the course landing page. However, all the reviews evaluated by Udemy's algorithm as 'authentic' and 'non-spam' get counted while calculating the overall rating of your course. You can view them in the 'performance' tab under 'reviews'.
But some reviews are appearing at performance section and approved and still not appearing in the course landing page, they have not selected any of them although they are approved!!!!
The reviews are approved that's good! But as I said, Udemy has its own algorithm to check if these are to be shown on the landing page. Please keep in mind, the users view the landing page for the purchasing decision they make, so its important that only the best reviews appear there.
By the way, do any of these reviews contain 'comments' with the rating? If not, they will not appear on the landing page. Saying this from my experience.