A note frack/chip (outside of something mechanical like a sticky valve) can be due to not hearing the note or interval clearly enough in your head and/or not buzzing the right pitch into the horn.
Something you can try is singing the note, interval or phrase that is giving you trouble (using a piano or drone for a pitch reference if needed) focusing on pitch accuracy. Then, after singing it a few times, move to buzzing the same thing on the mouthpiece. When buzzing the mouthpiece, I like to gliss to each note, so it sounds like you're using slide glissando on a trombone. Finally, play the same passage on the instrument.
Basically what the singing and buzzing both do is help strengthen your internal sense of relative pitch, so that when you're playing the instrument, you're more confident at how the music should sound. The buzzing has the added affect of helping you buzz the right note on the mouthpiece, which in turn helps your accuracy on the instrument. The mouthpiece glissando helps with ensuring your air stream and buzz is constant through an interval or musical phrase.
Let me know if you have any questions or if any of that was unclear/confusing!
Willson 2900 TA-1 Euphonium - Denis Wick 4AM
Yamaha YSL-643 Trombone - Bob Reeves BrassArk 5G "Gladstone"
Yamaha YSL-8440 Trombone - Denis Wick 5BS
VMI 3301S BBb Tuba - Schilke Helleberg
Past:
York Preference 3067 Euphonium - Denis Wick 4AL
Benge 165F Trombone - Benge Marcellus
Wessex BR140 Baritone - Denis Wick 6BS
F.E. Olds Special Trombone (ca. 1941)