Hi! Generally when the notes sound sharp, it's because on two reasons:
1) The blowing pressure is too high
2) The holes aren't completely covered
My suggestion to figure out if everything's ok, is that you start on the highest note (the high C on the second chamber) with all holes uncovered, and blowing really soft. If you're flat, then blow a little harder until you get the correct pitch. Then when you have that note correctly and you know how much blowing pressure you should be doing, you start descending the scale covering one hole. If you're too flat then it's probably because you aren't covering completely that hole, and you can start adjusting your grip until you get it right. And then you go down, one by one, hopefully completing the scale correctly until that low B on that chamber.
At first when I got my CODA I had this same problem, and that technique helped me realize how should I cover properly the holes, and what was the proper breath pressure.
I hope this helps!