Use Cargobob to carry vehicles?

I run into Los Santos International Airport and stole a Cargobob, after deploying the hooks by pressing right directional buttons ( i'm XB360 player ), I found that I can't grab a car on roads, even I fly real close to a stationary vehicle.

What did I miss?

I have done done this several times. It takes a bit of luck but the general technique I employ involves trying to swing the hook into the car. I have also managed to pick up cars that were not mine from the impound also, although whether or not they were another players or NPC cars I do not know.

Stick at it and eventually it should work.

Try pretty much landing on the vehicle with the hook out. The hook will attach in the middle of the target's roof, so try to be square with it. Practice on parked cars until you get it, parking garages are good for this. Once you get it it's easy. Hijacked an armored truck with it once.