I am going to give one example, but this happens at least 5-10 times on every turn and it is ruining the ability to play the game with any sort of meaningful strategy.
Specifically, I have large fleets set to Attack that never do anything, turn after turn. The fleet for this example is #1301326, but this problem is present for at least a dozen other fleets.
This fleet is comprised of 3 blackbirds and 124 cruisers. It is sitting at an enemy planet that is defended by a handful of 3-4 ship fleets.
This fleet never appears in turn logs. Not attacking, not defending, not "raining death from space" or anything else. It does nothing, turn after turn, even though it should either be attacking the small defending fleets or attacking the planet, however the game decides which it is going to do.
The net result is that I am attacking multiple planets, with multiple capitulation attempts each turn in the 20-30% range, yet the fleets do nothing, or have the occasional cruiser wiped out by one of the defending fleets. Never is it the case that the defending fleets are touched. Nor is the planet.
I am gonna go out on a limb here and guess this is not how the game is intended to work. Or if it is, it's some inscrutable combination of rules that I have been unable to decipher. In any case, the net result is planets that are impossible to take over and 3-4 ship fleets that are not only invincible, but also immune to attack in general.