EhEhEhEh! The problem is that your rpg/law isn't configured to be used against choppers in flight... Even if they're hovering. Notice the AI would use it against landed or low hovering choppers. Yet against flying choppers, the AI would only target...
So, all you must do is configure a new LAW/RPG rocket launcher with the capability of shooting down choppers. Compare the LAW config and the Stinger config to see what makes the Stinger usable against flting objects. Check the Config1.91 provided by BIS.