The first step is to get your sax checked for leaks. Once those are taken care of you can accurately diagnose any remaining problems with low notes as "pilot error".
A way to check the tightness of your embouchure from "The Art of Saxophone Playing" is to play 2nd space A and while you are holding the note with your free hand "bump" the neck octave key open and let it close.
- If the note goes to a high A that is flat and "flabby" the embouchure is too loose.
- If the note goes to a high A and stays for a while, the embouchure is too tight.
- If the note jumps to a nice sound high A and then immediately comes back down, the embouchure is correct.
A few tips I use with my students to improve playing the lowest notes include:
- Sing "AHH" on the lowest note you can hit, or do the first part of a yawn and play with this shape inside the mouth and throat.
- Blow lots of warm air.
- Keep the back of the tongue down (the "AHH" does this)
- Start on low G playing forte and quickly slur down to low C
- When low C sounds in the right octave, hold it as long as you can and repeat
- Repeat for low B, and Bb