+1 on the fork shaft pin shearing. If you are certain that the system is bled well, feels firm, and is adjusted, then the next most likely is the fork pin. Normally you will see this as a large amount of play when you go to adjust the linkage between the slave and clutch lever.
One note...the pin doesn't fail completely, but rather gets a spiral shear that wedges against the shaft, so the clutch still works...partially. A temporary fix is to remove the return spring on the clutch lever by the slave cylinder. This allows the master to pump more fluid into the system and possibly delay repair. Some on the forum removed the spring and were able to drive for years with a broken pin.