If you still cannot find, interact with, or send things to certain friends it may be because one or both of you does not have the latest version of the game. If this is the case, each of you should check the app store for any additional updates.
If you or your friend are a PlayStation user, it is possible the crossplay settings have been turned off for Sky. You should be able to correct this from the in-game settings: “PlayStation Network - Allow Cross-Platform Play”.
It is also possible that one or both of you have blocked the other accidentally. You should be able to correct this as well from your Friend Constellation menu.