Helicopters arent spawning?

[Chugster]

I found a few locations where helicopters are meant to spawn, such as Steel Point and on a building west of the Tower in Loren Square but there doesnt seem to be any vehicles there, are there any conditions under which choppers will or wont spawn?

 

[NOUS]

if you approach from above (in a heli or something) I think it doesn't spawn. So what I do is I walk up the stairs to the pad to make it spawn. Weird but works.