Two things could be wrong. You might be playing on a reed thatd too thick so you cant blow through your higher notes. which have a tendicy to play flat. Or, it could be that you just need to practice it. You may be biting down too hard or maybe too soft. My old sax teacher used to have me do this to fix my out of tune notes and find a nice rich tone. Ok, so play your high D and hold it biting down as hard as you can with a tuner in front of you. Now, loosen your jaw until you crack. Do that over and over again, trying to control where it cracks. Eventually, it will take a lot longer for it to crack and your range on that note will be huge. After you can loosen for a while and not crack, then find that perfect in tune spot. Of course it wont be in tune EVERY time, but now if its sharp or flat, you know which way to take the pitch, and will be able to do that and not sound liek your bending your notes. Good luck.