Ken Wollitz also has a nice description in his book about breath which amounts to fast air but without letting pressure build up behind the tongue. That has also been helpful for me. If I “panic” on run up the scale and tense up or try to force the notes, the notes above C are doomed.
Another thought, if you are getting the notes initially and then they fall apart, check for moisture in either the windway OR the head bore. I find that on my plastic YRA-312 (alto) the high D and F seem especially flakey when the instrument is wet. I end up needing to swab out the whole recorder in the middle of practice session before I can continue with those notes.