Cannot Paint on Mask

Hi there,

I have an issue I need to fix asap so I can continue my work on my honours project for university.

I need to re-texture a character rig. I can paint on my mask layer if I change the UV map of the character in Maya before exporting however when I do this, my rig breaks and his facial controllers bug out.

I need to be able to paint on masks in substance painter using the rigs original UV map. The map looks clean to me so I have no idea why Substance painter wont let me paint on it.

I am new to Substance and I need a solution asap.

Please note I cannot change the layout of the UV maps either as this too breaks the Maya rig.

Any advice is appreciated. Thank you.