Making matters worse, the shape of an existing reed does not necessarily reveal the steps by which it was produced. Cane deforms as it is scraped, due to the pressure of binding, so the final shape was created by both scraping and settling which is not always obvious to the self-teacher. So an isolated learner attempting to duplicate a working reed can be misled into scraping too much in critical areas. There are several main causes for difficulty hitting or holding the 2nd octave, in my experience. They are reflected in the topics shown below, which include remedies where I have found them, followed by some thoughts about preventative measures in future reeds.
Every reed plays best at some particular basic strength.
If its collar or bridle is set so as to open it up for greater loudness and strength than this ideal, the 2nd octave will become disproportionately harder to attain and hold. The first remedy to seek for any reed with octave difficulty is to simply close it down slightly. It may behave perfectly well, if playing somewhat quieter and sharper, when the elevation is reduced. Future reeds might be designed to play louder and stronger by leaving a bit more cane on the scrape in order to support greater elevation, and also perhaps leaving the blades slightly longer to compensate for tuning imbalance that would otherwise be created by a more open reed. A second common problem is that the bridle or collar may be at the wrong location on the blades, even if it is holding the elevation to the correct amount.