Artillery, I don't get this.

[Wdll]

I don't like to use artillery much, delays me and prefer to use the "money" on other things, so perhaps this is something well known to others, but new and unexplained to me.

Some times when I try to select units for bombardment (with a proper spotter or not), I don't have all my artillery units available, and by that I mean they don't always even appear on the B list. Sometimes they will be there, I will give order, then the next turn will dissapear and not fire or just not be there (on the list).

I thought it might be either something about having other units near, infront of them, hills or trees near, or just plain firing arc, but whatever I have tried I haven't noticed any change.

What might be interesting, I haven't noticed this while using my favourite artillery unit (PzH 2000), but I have with others (M109A*, 105mm M119 FH).

So, what happens?