Often difficulty with low notes is an air leak. Go down the scale till you hit one with bad rattly sound and then lay the sax sideways to see if the pad on that one is closing properly. It may be an auxilary pad further up the stack. Check if the auxs are closing properly. Should be a cork pushing them closed on a tab from another key. Sometimes the corks fall off! Sometimes a small leak can be countered by relaxing your jaw and pushing further onto the mpc. You can check a leak by covering the pad with duct tape.