That is often a very weak note on many instruments. I no longer have an Aulos bass or I’d try it myself. Is it poor compared with other recorders you have? Kenneth Wollitz in The Recorder Book goes through all the notes and comments on this one that it is “another rather weak note and may tend to be sharp” – if sharp, it can be brought down to pitch by blowing more softly. It might just be that this is how it is!