There are two possible reasons:
- You've reached the 30 friends limit from whom you receive the Energy bonus;
- You and/or your friends did not allow access to your Facebook friends list.
Both you and your friend need to give Football Rivals access to your Facebook friends list so the game can properly detect whenever one of your friends joins.
You can follow this guide to give access:
Also, keep in mind that simply installing the game is not enough. Your friends need to actively play in order for you to receive the Energy.