Typically, if you can't get notes out above D on soprano, its because you're closing off the mouthpiece. It may be that the tip opening is too small for how you are blowing. However, its also possible that you are biting as you get into the upper range of the soprano. I've worked with lots of students and its definitely a tendency on soprano to want to bite as you go up high, especially on soprano. This is not a good habit and only restricts the ability of the reed to vibrate properly. I would guess its the result of a combination of biting and not having the ideal mouthpiece.

     What mouthpiece are you currently using and what reed type and strength? If you focus on keeping a uniform embrasure up into the palm keys, does this help at all?

Cheers,

     Mark Overton
     www.saxquest.com