Hi Chief,
If you’re trying to get revenge, but the system keeps saying your opponent is online or behind a shield, it means they’re very active players. Accessing the Revenge function means finding the window of opportunity where your opponent is vulnerable to attack.
If the Revenge feature is unavailable altogether, it means you’ve already Revenge attacked this player. You can only do this one time for each village.
Additionally, you may have already attacked this village in multiplayer, preventing you from getting Revenge (it’s their turn for the revenge in this case).
Clash on!