Subsurface Influence Mask not working correctly

I’m trying to use an influence mask which I’ve made a texture for to control how much sub surface scattering I get on my leaves.

I’ve attached the images and as you can see the leaves are not affected by the mask at all.

I’ve tried inverting the mask as well but no luck.

Any help would be greatly appreciated.

Thanks
Ben

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What shading model are you using?