How can I improve my tone on the flute?

I have been using the "smiley"embouchure for almost 3 years and it has worked fine until now. The 2nd ledger-line C is the one that I am having most problems with. Has anyone else had any problems with this note? How can I improve my tone? When I play the 2nd ledger-line C it stays in the higher register for like two seconds and then it drops down to the 3rd space C and it sounds airy in both registers.