Weird Glitch

[Maka]

In the second mission of the mini dogfight campiagn, It says you have to kill 20 enemies. I've done this mission 4 times now and every time I get 19 and there's no planes left. Therefore time just runs out and I have to start over again. And unless i can get this, i won't be able to get collector or Ace Killer (I' have all the planes except for 2 and killed all aces except for 2)