I have 5 fleets at an enemy planet. Irrespective of size, they all have a 5% capitulation chance
What's in those fleets? I think if there's only 1 type of ship in a fleet, it's maxed at 5% chance for most planets. I've only seen single type fleets have over 5% chance on planets not classed as colonies (I think that's anything over level 25). If they're not mixed fleets, try using those. If they are, then my lovely theory is wrong >.<
and it seems only one of them is fighting, for all the others it says "planet is defended" ?
It's annoyingly always been like that. If they have 1 fleet, then only 1 of yours attacks, the others sit and watch.
