I noticed that only one sector of the planet was captured at any given time and it looked like it already had a match being played on it. Maybe having a full defender group but no territory for them to counter-attack is causing Call to Arms messages to go out to the wrong faction. It seemed like the Call to Arms should have been sending messages to attackers instead of defenders.
Edit: Wrong name of the planet
Edited by Domenoth, 23 April 2015 - 01:22 PM.