The mouth can be open too much and get stuck. Alternately, the mouth can sometimes be open too little, accompanied by a tight jaw.
The teacher could remind the student to open their mouth a bit and let the sound out. However, this may not be possible for the student to do right away. If the jaw is too tight and there is unnecessary muscular tension, then freedom of the mouth parts – the articulators – might take some time.
Most often, this condition would indicate that there is a problem at the sound source, and so the registration would have to be worked on so that the mouth parts could be freed of tension. Opening of the mouth would then be possible.
With exercises consisting of pitch, volume, and vowel that reflexively stimulates activity in either the arytenoid system or the crico-thyroid system or both, then incorrect tension is released. Included in these exercises can be certain staccato forms and consonant repetition that assist the freedom of registration as well as articulation.
When correct vocal cord movement is restored, rigidity releases, felt everywhere as freedom to move. The mouth is able to open naturally given the pitch and vowel and volume.
Functional freedom is always the goal!