Assume as you’ve fitted another shifter and this works friction between the inner and outer cable is not the problem.
I’ve had exactly the same problem which turned out to be muck and water in the X9 shifter just making the mechanism sticky. Plus there is a “tension” screw from the base of the shifter up through the body that needs to be correct.
They are a fiddle to dismantle, I took photos on my phone at each stage so I could refer to these and placed every component the right way up on a clean tray as they were removed.
Cleaned everything, light oil and rebuild and it’s been fine ever since.
There are a few youtube videos that show the process and I also found a lot of images just using Google and the image option that helped.
Another tip is that it is possible to convince yourself that the two main springs can be transposed, they can’t. Physically there is a fit but it won’t work. Taking out everything in order and will prevent this mistake.
Having done this a couple of times I would be happy to do the job again.