The setup of the front F is critical. If your front F key opens the F palm to it's fullest extent, then you will have more difficulty playing these (effectively altissimo) notes. Every horn design seems to have a particular venting for the front F that works optimally. The yamahas that I have addressed this on seem optimal at about 2-3 mm...as compared to Yanagisawas that seem to prefer just a crack and Super 20s that are fine with nearly full open.