I'm a flute noob, but will not be able to get instruction until January. My background is Jr. High and High School Band, Clarinet, six years total. Over 50 years ago

My instrument is a Yamaha 225S, with a complete technician repad/rebuild, newly done. The book I'm using is Essential Elements for Band, Flute Book 1. I'm working through it, playing a minimum of 1 hour daily.
My question is regarding lower notes, starting with D and below. My notes have no clarity or volume, they sound like a very breathy note blown over a bottle top. Is mastery of these lower notes something that comes with time, or is there a problem with my form a teacher will have to correct?
Thanks for any feedback.