I was attacking for the first time a blue black hole that required specific ships so I sent the required minimum. Then I realized the number of required ships increase every 15 minutes so I recalled my attack to add more ships since it was going to take 45 minutes to get to the black hole. To my surprise the ships in the cancelled attack did not go back into my attack wave bank so I lost the use of the ships for further alien attacks. I was also unable to reattack the black hole because it said I had already attacked it. No emblems or spice either since there was no attack made.
This seems to be a bug. I would think that if I cancel an attack, the ships sent should go back into my attack wave for future attacks. In the meantime, I guess I will just have to remember to not cancel attacks and just see if there are sufficient numbers when the fleet gets to the hole.