I hope it’s okay i’m digging up this topic, but I have exactly the same problem. And I have made sure the null object is a 3D layer, that the comp is set to active camera, and the camera is parented to the null.
I was following a tutorial for motion graphics/transitions. I have two clips lined up on top of each other, wanting to pan across them. I moved one of them to the right in the 3D space while the other one stays where it is. Then I moved the Null object, so it pans across. The null object is moving, but the camera isn’t.
I took a screenshot of my project, here.

I have no idea what I did wrong.
Thanks in advance!