An open hole flute has keys with holes in them. If you don't cover the holes on the keys, you can't really play very much.

Um...I say go take a lesson with a flute instructor in your area. That would be your best bet to figuring out what's wrong. Pay close attention to what it feels like when you finally get F and Eb out...once you figure out what it is that you are doing differently, try to imitate that every time you attempt it.

Which octave of F and Eb? If I know the octave, I may be of some help.

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