In one of runs, I killed everyone without the Void Worm spawning, then was able to walk around freely. Once I walked out into the open, the Void Worm spawned. I think it's an area trigger. If you don't walk into a certain area, it doesn't spawn. So, if you fight solely from the sides, it won't spawn. I can't currently test this hypothesis, though.
I marched my party all over the area attempting to get the Worm to spawn. No luck.
I have had what you describe happen in my runs on the last build. The Worm would pop up quickly after combat with the humans ended. This time he is nowhere to be found.
