Not sure if this has been mentioned, but I have discovered a possible oversight and/or bug with the last update. The Capital ship challenge to 5* your capitals is currently unplayable. I have put in a bug report, but posting it here as well for max visibility.
The Challenge requires 6 5* non-capitals and after the update you have 3 ships starting line up plus the 2 reinforcements for having a 4* capital for a total of 5 ships you can send into the battle. This effectively makes the battle unplayable, because the battle will not start without the required 6 ships in fleet.