How do I help my branded Uruk captain bully Uruk soldiers into joining him?

When the uruk captain starts to "bully" other uruks, go into wraith mode (LB on xbox controller IIRK). You will see that some uruks are yellow coloured. Those are your targets.

Kill these yellow marked uruks, and you should complete the task.

Note: The captain will go to other groups to bully them, so follow him.

Note 2: Don't dismiss the possibility of a glitch. The branding could have caused one.