my middle notes sound too airy

[adult_learner]

I have no problem with high and low notes, but my middle notes sound too airy. What causes this? What should I do to fix this problem?

[vandoren]

If by middle you mean D2 -> F2 then it could be that you are tending your embouchure to the lower register. I find these notes the most difficult to foster a good tone especially E2. If I mouth them more towards G2 then I find a better tone. Good luck

[flutepicc06]

What causes this? What should I do to fix this problem?

Without being able to observe you and your playing, it's very hard to know what exactly might be causing this, but the solution is quite simple....Go find a teacher and start taking lessons! If you're not already studying privately, the odds are very good that you've developed some bad habits (probably even a few you haven't noticed yet). Depending on what they are, you could risk serious harm to your hands/arms or jaw, which can end your playing for good if they're not corrected. If you are studying privately, discuss this with your teacher and see what they have to say.