Take a look at the linkages to the octave mechanism. See what is moving and what is not. It may be that a cork is too thick under the octave key, or one of the linkages is bent. It may be a missing a cork. If something is bent you might have to take it off the horn to straighten it to bring it back to it's proper positon. BE CAREFUL, brass is forgiving but within reason. With some bends it is necessary to heat the brass b4 bending. If one of the linked pieces to your octave is bent or twisted it is already work hardened, brittle, and it might be ready to break, so again, be careful. You should be able to see if some part of the linkage looks twisted or bent.