There can be a number of different reasons as to why you are unable to add or see specific Facebook friends.
The most common cause is that your friend's captain is already being used by the maximum number of teams - so it's worth getting them to check by asking them to navigate to the Captain Teams screen (Main Menu > Squad screen > Captain Teams). A player's captain can only play for a total of 10 other Facebook friends.
The second most common cause is that you have reached the maximum number of Facebook friends playing the app that can be shown at any one time. So regardless of whether or not your friend has room to add you or vice versa, if you have more than 200 Facebook friends playing the app, not everyone will be shown when scrolling your list. You'll need to reduce your friends list.