Ah, see I have all the enemies pre-placed, not spawned. I assume then that I don't really need a container.
I figured it out, though.
I just used the Event System > For Each > Body, then the Action Pin Head to Body.
I wish I knew exactly how it was able to determine which head to pin to the body, but none the less it works.
Edit: It seems I do need the container. That seems to have been what was doing it; it preferred the head inside it's container when attempting to pin one on.
Thank you for the help, both of you.