I didn't get Silacrum to spawn either. Cinis spawned as well, I flew under the wings (twice just to be sure). Maybe its a specific spot or specific wing? That's all I can think of.

Which one did you fly under? I flew under the right-most wing if approaching from the Podarge's (the ones sitting on the water, not the ones that fly in) front.

I flew under each wing, by the cabin and further out from the cabin on the landed Podarge's after a second run through, still no spawn.

I flew under the parked ones AND the ones flying away. I had both spawn.