Material not changing roughness

I have a material function that I use to change the roughness of my materials. This has worked with every other material except all the barks of my trees. Here is how I have implemented it.

All the (mf_global) does is subtracts the roughness from a given value. This method has worked on every single other material except for the ones used for the barks of my trees.

Any suggestions?

Maybe it could work better coming from the red channel rather than the rgb channel

Unfortunately, there is no change even from the red channel.

This method has worked on every single other material except for the ones used for the barks of my trees.
What is the difference between materials ?