I have stationed 3 formation (2apc and one for class barrack) and the troops without formation in my..

I have stationed 3 formation (2apc and one for class barrack) and the troops without formation in my defense building. When I was attacked Only 2 formations and the troops without formations participated in the defense of my city. Why the 3rd formation (from class barrack) did not participate? All the troops were stationed in the city and defense building when the other player attack me. Do you know?