Summary Low notes are a problem for the saxophonist. It is important to keep the instrument in good mechanical order since even slight leaks create serious problems.
Reeds can also be part of the problem.
A soft reed will assist the response of the low register but may make higher register playing more difficult. Try to compromise with a somewhat softer reed that will still produce a good tone in the upper registers. It may be helpful to use a reed knife or Dutch rush to adjust a reed for the best performance. A poor embouchure will effect the low tones the most. A firm but not tight embouchure will let the reed vibrate freely in all registers.