get a sax light, then you will see the leak,, you should be able to subtone the D,s first and second octave, if that works could be the octave key,, but could be another, or your just throating the second D wrong and jumping down ,subtoning the second D and thinking its the low D,, even though you use the octave key you still need to adjust the throat and some air to accommodate the second octave, my thoughts anyway
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