I recently acquired a concert flute and started to play with it, getting used to the embouchure and properly making a sound. I reached a certain stopping point when trying to learn all by myself, though. I can make all the notes from the first octave sound properly, with a couple of tries, except the the ones ranging from D -> C# -> C. I blow the flute but the sound is fickle, and very, very quiet, I can hardly "spot" it over the sound of me blowing across the little flute hole. Am I missing something when related to embouchure? Should it change to properly address the lower notes on the instrument? Maybe I need to direct the flow or the speed of the air differently?
A "real" flute player might have better insights, but you are certainly not alone in having difficulties with the lower notes. A few things to check: