It seemed to me as a starter that the middle notes were fine, say from low Eb up to high Bb, but anything outside this range needed work - one day you'd find you could hit high B or C consistently without squeaking, a week later D, then Eb, E..... it just takes a bit of time for you to increase the notes at the top and bottom ends that you can get to without thinking about.
The bottom notes are a different matter, the most common problem I had with them was stopping them from jumping an octave (or some other harmonic, like a fifth or even a twelfth), but again it's just a matter of working at it - also, worrying about it can cause you to subconsciously tense up to the point where the duff note or squeak will happen, so again it's just a matter of trying to keep relaxed while you play the high (or lowest) notes, and it'll happen soon enough.
Cheers, Bob