There is a problem with this game Steve. For some reason my guys on opportunity fire are not firing. I lost one and couldn't understand as I swore he was on opp fire. But let it go. But now I have lost another two, both were definitely on opp fire and did not fire at all despite the enemy being straight in front of them and able to basically walk right up to them. Stood like mannequins.
One would have been an enemy walking through a door, the other two were my men stood in corridors. Enemy seen but on the log no firing except from the enemy.
Probably they have a visibility issue. When a friendly unit is standing 1 square away from the prepared shooter and he is visible to the shooter, he will not shoot. Another reason is a hiding unit. When you have a non visible unit in the way of a visible unit, opp fire will not happen, because the opponent is not visible. The least probable is the shooting visibility vs sight visibility which is different. They cannot see each other, but they can shoot each other. It usually happens in the other direction and the opp. fire is shooting.