Must be device specific, I got to 4th mission on my iPhone no problem, now with my s4 under same account I cannot get to the 4th mission period.
I have landed back at my base, enemy base, even after reloading and re bombing enemy base several times (yes, hangars respawn) I still can't get a mission complete.
Now this is a minor issue to me, I would rather be flying against other people, but I also would like to complete all the AI missions once just to say I did and see what they are like!