There are a couple of possible things going on here.
Physical :
Improper chop setting (too tense creates a seal that will not let air out)
Not timing the elements of the attack together (breath, tongue,air support, air flow)
Sensation (the student does not yet feel the note but relies on sound to make the proper pitch.) A drill combining sight singing and playing can help here.
Mental:
Nerves (I've seen people who were so concerned about being wrong that they could not play a scale in front of another person.) This is a tough one and takes a great deal of time.
Being unsure (A sensation drill can solve this.)
Both physical and mental :
This is most likely the case here. Work on the sensation drill and on setting up drills. Play a note remove the horn from the lips. Then reset the embouchure and play a note. This can be used to play hundreds of initial attacks and will lead to more confidience.
'Pops'
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Pops