i would agree with reactolite

(i too have a yamaha 211sII) bought cheap

and used
i couldnt even hit low c on my 2nd hand flute when i first got it.
as people have siad, best to get a flautist or two to check it out(which is what i did)
they could play it , but commented the flute was a little funny.
took it in for a major service, turns out, i had 2 leaky pads, a shrunken cork, unbalanced keywork, uneven tone holes, and bent keys
i mean, i could get sound out fo the flute mostly.. but it must've been leaking all over the shop, which is why i couldnt hit quite a few notes
i can hit low C now, but its still difficult.
so id say, it could be just the flute

maybe see if you can borrow someones flute and try low C

?