Hi, Kate!
Don’t rule out a leaking tuning slide, either (look for moisture buildup on the slide after you’ve played it a while; a lot of moisture is an indicator it’s leaky). If that’s the case, Teflon tape (thanks, E!) or beeswax/linseed oil (you can make your own block by melting beeswax with a few drops of raw linseed oil and letting it cool into a block – thanks, John!; I poured mine into an Altoids tin and that works great as a container) or a good greasing (check your maker’s instructions about whether it’s supposed to be a dry slide or not first, tho’ – thanks, Hammy!) can help there.
And plumber’s teflon tape wrapped thin on the joints can help, too, especially now that it’s winter and your flute is drying out in the forced-air heat. Are you keeping the flute in a 60%+ humidity box?
Finally, hair-rubber bands on your keys can indeed be your friends. The Eb and F keys seem to be my biggest “trouble children” across the board.
A side note: If I’m in need of them, I make sure to remove all Teflon tape AND rubber bands after every playing session. Don’t want the wood to swell for some weird reason and be taped or banded down so it can’t expand!
My experience is that my flutes want to shrink pretty much all over in the winter and dry air; the ones made in Ireland/the UK suffer the most – I guess because the wood was seasoned in a more humid climate?
FINALLY, a tiny crack, maybe hiding under – or at the edge of – your tuning cap? is not out of the realm. Do the low notes get better after you play it in for, say, 30 minutes? Could be that the wood expands enough thru your warm breath and its resultant moisture buildup to help close a hairline gap somewhere. (Sometimes a fresh coat of oil can also clear up a flute’s playing in this situation, although only temporarily and it’s not good to oil too much)
From what I understand, a lot more flutes probably have wee cracks than many of us know, and the winter is when they really show up.
(And of course, there is the moisture-buildup thing mentioned earlier, too, but that’s cleared up with a simple shake, blow-out or swabbing)
(Oh, and if you use cigarette papers under the keys (I do it, too, duuuuuude
), make sure you don’t get the gummed side under the pads.)
Anyway, that’s my experience with these crazy things so far – just remember that wood is an organic material, and it changes with the weather. Good luck!
Cat.
P.S. And if none of this works, check the placement of your right &/or left ring fingers; I can lose those notes if I get lazy there.