How much are you missing it by? - is the boss clearly a different diameter, or is it just a matter of a tight fit? I recently had the same problem on an older bike, and in resolving the problem I learned that it is not uncommon for the bosses to mushroom out a bit under the pressure of the fixing bolt compressing in on them. The LBS had a small cutting tool/mill that we fit over the boss and twisted it down the length of the stud -- it mills it back to size. Worked like a charm and the canti arm slid on with no problem.