One joint in a robot cannot be controlled. Any hints?

Hello! I am trying to use Isaac Sim with the Fetch robot.
I can use the Articulation Controller to control the arm.
However, I found that only one joint “head tilt joint” is not responding to the controller.
The other head joint “head pan joint” can be controlled.
Could you please provide any hint on what can be wrong?
Here is a screenshot:

The problem is solved if I check “merge fixed joint” in the URDF importer, but not sure why.

Hi @yuxiang - This document will be helpful to you: Omniverse URDF Importer — omniverse-urdf-importer 1.1.4 documentation