any tips on getting a good low C?

I’ve recently started playing an eight keyed flute but am having problems getting much out on C nat, C# is okay (ish) but for C nat there’s nowt coming out. I don’t even know where to begin to get the note as i’m not getting even a hint to work on. I’ve checked that all the other keys are securely covering their tone holes. I would appreciate any advice you might have.

Ooops I found the problem it was the pad on the C nat key (I should have checked it earlier - I just assumed it was making a good contact). I just had a friend push down on it whilst I played and out popped a fully formed C nat!! Sorted.