Re-ordering JVBs for round-robin selection

Dear all,
I have several JVBs connected to the same nginx/prosody/jicofo instance. I observe that it is always a fixed JVB that firstly serves meeting room, then the second JVB serves the second meeting and so on. The order seems unchanged.
Is there any setting to determine/change this order ? How does the round-robin algorithm setup the pool of JVBs (they all are idle at the beginning) to pick up one after the other for serving the coming meeting?
Many thanks