Can't get ground AAA units to attack me as I pass by in the air

[Stone]

So, I'm somewhat new to the DCS mission editor, but have created some missions successfully. However, in my most recent mission I've placed several WWII AAA units on the ground expecting that they attempt to shoot me down when I pass by (P-51D). But instead, they just ignore me, "unless I fire at them first". I haven't had to do that in other missions I've created. Is there some things I can look at to see what's different this time? Thanks in advance.