if you've learnt how to direct the airstream across the embouchure (first octave) and blowing up into the embouchure (higher octaves), then you need to learn how to blow into the embouchure.
Make a big O with the back of your throat, and blow in for the low notes. Compare that with your usual technique.
If it works, then use the 'abdominal punch' technique. Force out your tummy so that it's like a belly pop very suddenly. That will punch the air through big O in the back of your throat to give you the deep notes.
If that doesn't work, then you have an embouchure problem. Perhaps you aren't covering the embouchure sufficiently. It is easier to 'grunt' into the embouchure, covering most of it, than it is, to wisp whistle using a light touch of an embouchure cover, for the low notes. If you vary degrees of 'swallowing' up the embouchure until the mid-line, and rotating the headjoint (below/above airstream plane); angling it, and combining the above techniques with practice, provided your flute is serviced and pads all sealing, you should be able to get it by the end of the day