Hi there, I assume that the problem could also lie in the diplomacy settings (i.e. "Are they really an enemy?"). When setting up the match, make sure that their team number is different from yours (or there can be no team number assigned to both players). If the diplomacy mode is enabled (meaning if it's allowed to change your teams during the game), you can try opening the diplomacy setting (icon somewhere in the top right corner) and check if the AI is set to Enemy and not Neutral.