Non Firing Artillery

[Gordons HQ]

I've been playing these games quiet a bit now and I'm often frustrated by lots of artillery which for some reason or another is unable to fire. In for instance the Normandy campaigns this is compounded with lots of ships just stood by doing nothing while the infantry struggles onshore.
I can understand moving artillery units, then having to set up again and sometimes breakdowns in communications. But surely in 2 hour turns there is a pretty good chance the unit will be ready to fire and in communication.
Lack of ammo might well be another reason, but I wouldn't have thought that would apply to ships that carry large amounts of ammo.
I wonder if anyone can enlighten me on the reasoning behind this non firing.

Whilst on the subject of artillery the Commonwealth forces have an abundance of 3" mortars in Bren Carriers yet when firing these, they never seem to get any result, or very occasionally 1 man. Why are these so ineffective even against men who are not dug in.