So I've noticed in some of my previous games that when enemy aircrafts get detected by my SAM sometimes it doesn't fire immediately. I also noticed that this happens when I stack SAM with other units like armored units. They only attack( or more like defend against the aircraft) if that stack is attacked directly. So when enemy aircraft fly in I have to separate the SAM from the armor so they attack. Is this a bug or am I missing something about which units I can Stack SAMs with?