It took me a long time to get those kills as well. I found it's mostly controlled by having a team that wants to shoot down helos. If you have a few engies working on the same helo, it becomes a battle of being the one to get the killing blow instead of just shooting them down.
As a note, if there are still flares in the air, or another enemy hits flares nearby, the rocket will go after them. Even if you have a lock on the helo or jet, it will still go after active flares after being launched.
But, yea also it just misses sometimes. I mean it's good it's not completely OP like the stinger was in older titles for a time, but honestly it feels like the best anti air is just a plain RPG. Kinda defeats the purpose of the stinger I feel.
