Hi,
I was looking at Weissenborn's Practical Method for the Bassoon and the fingering chart at the beginning of the book says the bassoon can go all the way up to high F at the top of the treble clef. I have seen people such as Kristian Oma Rønnes go even higher on Youtube, but I myself cannot do it. I can get the altissimo Bb and B just fine, but C is kind of hard and above that it is almost impossible to make a sound. I play on a bassoon without a D flick/vent key and with medium-hard reeds. Are there any tips to get a sound up there?

Thank you!