The problem is that the trigger only fires on machines that are player squad leaders, due to the trigger conditions. Thus,
every squad leader will see messages as though they had requested artillery, even if they weren't the ones requesting fire support, or even if they were on the same side as the one that requested. Noone else will see messages at all.
This artillery script works well in SP, but
really gets into trouble with COOP or MP. The way around this mess is to create local triggers via scripting and broadcast the messages in some way. You also have real problems with concurrent fire requests and the fact that anyone can spam fire down to complete a mission. Really, it is a lot more effort than it first appears!
I'm currently working on much improved (configurable, COOP/MP/JIP compatible), version of this script, but I'm rather busy with other scripts, so I probably won't be releasing it very soon (Look out for SPON Artillery...someday). Sorry! So far, I have it working very nicely in COOP/no-JIP, so I still have a fair amount of work to put in. Since noone has put you out of your misery before now, I assume that such a script doesn't already exist, or I'd save my time and effort ;P
Plus I am unsure if the shells land in the same place for all players as of the random shell spawning?
The westAri script only spawns the shells on the server, so they affect everyone exactly the same.