I sent my course to friends and colleagues and my LinkedIn followers. Many of them have posted a review almost a week ago but I cannot see almost 50% of the reviews. Is their some sort of policy of I am missing something
If they just left a review as a favor to you without actually watching the course, Udemy's algorithms would rightfully flag that as a "fake review" and remove it.
Be careful to never explicitly ask for a good review from anyone, as that is against policy. See https://support.udemy.com/hc/en-us/articles/360007512653-Rules-Against-Manipulating-or-Influencing-t... for details.
everyone who asked me back have actually completed 100% of the course and still not showing up. Then some users have done 20% and their reviews are showing up. So not sure what I am doing wrong or is this some sort of policy
Yes actually there's a policy of udemy where they check all the reviews for them to be genuine so it may take time to show up on your course but will hage their effect.
Don't worry it will take time but they'll show up.
All the best