No Fly Zone

I’ve done “Halo” about 3 times in a row, and NONE of those times were there 4 banshees. I kill the first two at the beginning of the level, and then don’t see any for the rest of the level. What’s the problem here?

There should be two derping about in the sky in that hub area where you search for the remaining marines. They’ll only appear after you’ve completed one of them. Everytime I do the cliffside area first they appear when I exit into the hub area.