Is it possible that you actually mean the low C# key and not B key? If that's the case the problem makes a lot of sense. If you really mean the B key then maybe just when you press both the B and Bb keys for some reason you don't press one of them (the Bb key) enough. If you press the Bb enough, adding the B key will do nothing that would create a leak. Unless pressing both the B and Bb key somehow makes your hand twist in a way that another pad doesn't close.