Call of Duty Black Ops- Can't unlock "destroy 30 aircraft" challenge ghost pro?

ok .I have completed the 20 kills challenge and destroy a sentry gun challenge.But I can't seem to complete the destroy 30 aircraft challenge.I have equipped the ghost perk and an m72 law launcher.Then in the game when I see a spy plane or anything else,I destroy it successfully.So far I have destroyed over 30 spy planes but when I go to the ghost perk challenges, It says destroyed 5 out of 30 aircraft. I get frustrated trying to destroy them for nothing! Please answer.I really wanna know why this happens.thanks