Hi, I have been browsing since I restarted playing flute a year ago. I am currently doing ABRSM Grade 8. I play on a very nice Pearl Dolce but recently upgraded the headjoint to a seamed Altus headjoint. The flute plays very nicely except for just one note c3!! I feel like just that note needs a tighter embrochure and I can only play it loudly. I can play up to G3 at pp and p volume, except that note. I can only play A3-C4 at louder dynamics, but that's just probably need a bit of tone study.

But I am truly stumped by being unable to play C3 softly, or even sustaining that particular note

When you say C3, I assume you mean the 3rd octave C (one octave below the highest C possible). That's actually C6 in octave counting notation (C4 is middle C on piano, which is the lowest note any flute with a C foot can play). I would say practice an overtone exercise until playing that C becomes easier. Start on the low C (with the roller) and keep blowing overtones without changing your fingering. You should go from C1 to C2 to G2 to C3. Try holding it out with the overtone fingering, and when you switch back to the real fingering, it'll be much easier to play and sustain that note.

Also--you can continue the overtone series exercise if you want to REALLY make the C easier. The next note up is E3, and then G3.

Just to rule things out, have your teacher play it, and make sure its not your flute, rather than your technique.

Thanks for the reply. Tried the harmonics exercise and would say it worked, and I can sustain that better. Just can't play it softly yet. Realise I have similar problems with the c#above it too (the note is suddenly loud and more difficult to play than the surrounding notes below and above it)

Is that a common problem? After all, c and c# are the notes with the least number of keys depressed in the high register.

One issue with that note is that the tube is shorter and because of this, the tone coming from the first open finger hole will make it seem louder to the player.
Another issue is that the diameter of the tube for such a short length and the embouchure hole size makes it acoustically more difficult to control.
Practice your long tones for control and focus. :-)