Three things come to me. Either the bar is the wrong way around, it’s damaged or the endcap is loose. Have you tried watching one of their YouTube videos to fix it? 

(302) How to replace the brush bar on your Dyson V6 cord-free vacuum's motor head - YouTube

If you’re following the instructions to the letter, as they do in the video. The only option would be to try and replace the bar.  

Hi @delicategenius 

We are sorry to hear that the brush bar stopped spinning, to protect the motor the brush bar stops spinning if something becomes tangled around the bristles at either end of the bar. Removing the brush bar and clearing it of any hair and debris should solves the problem and we do not recommend cleaning any parts with water besides the filter.

As you have checked for any obstruction, this could be an indication to debris trapped elsewhere in the machine, interfering with the electrical components.

The easiest way to identify where this debris may be, is to visually inspect each part that can be removed and connect back. Start by removing each component from the machine and inspecting it for trapped debris.  

Be careful not to pull the power trigger while inspecting the machine. 

Make sure any debris is removed and all parts are refitted before using your machine. 

Further instruction on checking for a blockage, including a step-by-step guide, can be found through the below resource. 

1. The machine user manual. 
2. Dyson’s online support.
3. Dyson official YouTube channel.

Hoping this helps. 

Over helpful sharer here: Give this link a read over - You’ll find your answer I’m sure.