The stoppage is either a mismatched or kinked guide tube. The cable feeds into a tube attached to the underside of the sunroof. That tube slides over (or into, I forget which) a second, fixed tube attached to the inner fixed roof.

When you get your sunroof headliner out you can squint up into that area with a flashlight and clearly see the arrangement. Be careful with the fixed tube. If it gets bent, the process to replace it is really painful.