C-Slaves not getting the buffs of their Communion Masters?

[tinkthank]

I have been watching the replays of my Pythium battles to try and figure out how I can optimize the ratio of masters/slaves, and often click on my poor Theurg Communicants to see when their fatigue gets too high.

And I see that although a master will cast Body Etheral, many slaves wont get it, while others do. Ditto for Air Shield and Personal Luck, which I have scripted. I have, say, about 3 Masters and about 20 slaves, and some of the masters have different scripts (usually variations on a theme: Quickness, Master, Luck, etc., but one will cast Divine Blessing first while the other cast Master first, etc.).

I thought this was "broken", until it occured to me that maybe I misunderstand the whole Master/Slave thing:
I had thought that when my Theurgs cast Communion Master, that they all join one large communion with all of the Communicants around (and the Acolytes who have cast Slave).
But it *seems* as if there are instead 3 different communions going on, and not all slaves are slaves for all 3 masters.
Is this the case? And if so, how is it determined who becomes master for whom? Any tricks I should notice?

thank you