To answer your question:
"...how often or what would cause the balloon not to work? "
Plenty. Muscles, tendons, ligaments, nerves, glands, arteries, bones.
This is the problem with this whole CCSVI thing as I see it. While less than perfect blood flow may be a problem for MS as well as a host of other diseases, this whole ballon / stent approach addresses only the symptom, not the cause. Stenosis of a vein MAY cause a problem with blood drainage and flow. However the next question to address BEFORE doing anything is to determine the actual cause of the stenois.
There is a wonderfull image on Dr. Haacke's
site (Case 3) that shows stenois of the jugular caused by a bone overgrowth adjacent to the vein. All the balloning and stenting in the world will not correct that problem for very long.
Some people with stents - which for the most part were designrd for use in arteries - complain about pain. The interesting thing about that is that veins have no sensory nerve endings. They cannot 'feel' anything. Any pain felt therefore must be caused by pressure being placed by the stent on tissues outside the vein.