Well, I think I've diagnosed the problem, and it has nothing to do with playMaker.
I set up a clean project, excluding playMaker. I made two simple scripts (from this YouTube tutorial:
), one which instantiates a missile, and one, on the missile itself, which positions itself and guides itself to a preset target--a simple homing missile.
In the tutorial the instantiation script, attached to the Main Camera, instantiates the missile on the Main Camera, and the homing script, attached to the missile, guides it to the target.
In my version, the instantiation script, attached to the Main Camera, instantiates the missile at 0,0,0 (just like the BowLauncher script had been doing in my game scene), and the homing script, attached to the missile, causes the missile to vibrate back and forth, going nowhere in particular.
I don't know what's wrong yet, but at least I know it isn't playMaker, so I think I'll go on over to the Unity Forum.
I'd mark this question "solved", but I haven't figured out how yet.
Thanks to everyone who has taken a look.