There is currently a limit of 151 friends which you can have in the game. If you have over 151 friends who play Coin Master, some of them will not be displayed. For this same reason, you may be able to see a friend in your game, but they would not see you.
You can remove friends from the game by unfriending them on Facebook, and after this has updated in the game that friend will randomly be replaced by another.
We are working to increase the number of friends available in the game. Stay tuned on our Facebook page!