I've just bought a used Blessing XL in reasonably good condition. With my mouthpiece - a cheap but perfectly reasonable (at least in other cornets) Gewa 1.5c short shank - it's fine from D upwards, but low C and the notes below just don't seem to want to play. With the slide pulled out by about an inch it all comes right. The cornet is a shepherds crook design. I've diagnosed that I need a longer shank mouthpiece to make it work, it being a US cornet. Am I right?