adjustment slider not working

Hey @Asillett. I'll need more info to help you figure this out. Please share the version of Lightroom & screenshots of the interface at your end.

In Lightroom Classic, the masking sliders & the global adjustment sliders are just a scroll apart, which could sometimes be confusing. You can check this tutorial for Linear Gradient to be sure that its not a user error you're running into: https://helpx.adobe.com/lightroom-cc/how-to/linear-gradient-tool.html

Let me know if this helps. Thanks!

Sameer K