New to the game - problem...

[bigbossmalone]

Hi all
I've recently started playing Theater of War, just completed the training missions, no problem. My first campaign mission, however, is giving me a bit of a headache.
Playing the Polish campaign, the first misssion is defense of a town.
To the rear of my trenches, there are 3 artillery guns. The problem is that these guns refuse to fire. I give them Area fire/Attack commands, and they do nothing.
The outcome of the battle, as a result, is a swift defeat for me.
Not the greatest way to start the game.
Does anyone know what conditions must be met for them to fire?
As I said, this is putting me off a bit, and I'm not even sure if I want to bother...
but, if anyone can give me a solution, maybe I will persevere and can still get to enjoy the game...
Thanks in advance