It is normal for the A and Bb throat tones to have a thinner sound. There may be a touch of fuzziness. Instead of the register key for the Bb, one can use the third-up (2nd from the top) trill key, but this is very awkward. Most professionals get very good at using resonance fingerings. Here are my favorites. How they work will depend a LOT on what clarinet you play. These work on Buffets for sure, but on many others as well, especially those that have a similar medium bore.
For playing at a forte volume:
A add the first finger on the Rt hand.
When playing softer:
A add L 2+3 R 2+3 and the F/C key.
The Bb may require more lip pressure and these two fingerings will need to be tested to see at what volume they will be useful.
L 2+3 plus R 2+3 I like using this fingering with the top A fingering above, because neither require the pinkie F/C key.
or L 3 plus R 3 plus the F/C key. This resonance fingering is the standard favorite.