When I try to unmask on the mask layer using a white brush, nothing happens. However, masking with a black brush works perfectly. I am using PS on Ipad Pro 2020

Check the brush blending mode (under the three dots for more brush settings). Set it to normal.

https://helpx.adobe.com/uk/photoshop/using/photoshop-brushes-ipad.html#modify-brush-settings

Dave

Hey, ​@pepot_23. Welcome to the Photoshop Community. I'll need more info to help you figure this out. 

I’ve just tried this and couldn’t get the mask to work. Could you please share more details, such as the version of Photoshop, iPadOS details, and screenshots & video recordings of what you see? 

Thanks! 
Sameer K

(Type '@' and type my name to mention me when you reply)