Hey @Arthur Ruminant.

It doesn’t look like any of our other members have experienced this issue, which can be seen as a positive. However, has mean you’ve waited some time for a reply. 

The max slider should be easy to operate, if it is not moving or stuck in position, with no clear reason as to why, then it could have developed a fault. In this case, you may require a replacement cleaner head.

Before you look at a replacement part, can I suggest you check for any blockages or damage inside the cleaner head, ensuring the brush bar and casing is is free from obstructions. This video should help - How to reset the brush bar on your Dyson DC41 or DC43 vacuum 

if this hasn’t solved the issue and you need to order a part, visit our How to: Order spare parts and accessories post. This walks through where you can order replacement, ensuring they are compatible with your machine. 

Thanks. 

Max.