As Greg/Wolfgang suggests, the welds can break on the clutch cross-shaft. Clutch operation will be gradually affected if only one weld breaks, and completely cease when they both break. Look at the picture Greg posted on the right with the red lines. The weld breaks at the joint between the shaft and the throwout bearing arms.
When rebuilding a transmission, the clutch cross-shaft should be checked closely and the brass bushings replaced and greased. This is an often forgotten or missed step, besides greasing the clutch tube when replacing the cable.
Also pay attention to a proper arc in the Bowden tube, that can affect smooth clutch take-up and make it seem like you don't know how to use a clutch.