There is currently a limit of 151 friends which you can have in the game. If you have over 151 friends who play this game, 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.
However, there is a way to fix this problem: You can remove friends by deleting/unfriending/removing them from your Facebook. The game will refresh the friend list and another friend who play this game will be randomly replace the removed friend.