Low B-flat when hitting low B issue

It may be that if the finger is contacting the edge of the Bb spatula near the B key touch that you are not getting the full leverage of the key to close both the large pads on the bell. If you have a way to put a light into the bell and press the B key very lightly followed by the Bb key, you can quickly tell if there is a regulation or a pad closing problem. To cover the tonehole perfectly with just the lightest touch should be the standard for those notes if you have a professional model sax.