Why does somebody undertake a long-distance instructor course?

This is a hard one to answer because none of the answers really make any sense.
The most obvious answers are not very flattering:

1. Lazy
2. Stupid
3. Ignorant
4. Arrogant
5. No discipline
6. No credibility
7. No self-respect
8. Lacks the necessary competence
9. Likes the idea of being a tai chi instructor
10. Incapable of completing a genuine 'real world' instructor course in a known school

The sad part is that there are organisations out there profiteering from tai chi deadbeats.
This is why tai chi has no credibility in the 21st Century martial arts world.

Beginners treat tai chi and baguazhang like external systems and rely upon deeply bent knees and exaggerated stances for power.
Their seeming root is accomplished through physicality not energy.
The jing of 'root' is created by mind, by energy, not-doing, by allowing - not by squatting.

If a student is failing, then demotion is a sensible option.

Usually, moving to a lower grade serves to remove any pressure from the student and provides an opportunity to work on the appropriate material.

The student may resist the demotion; but this is not constructive.

It can be easy to fall into external habits of doing.
i.e. physically dropping the body... instead of mentally dropping, and making only a slight physical action.

A common habit is the degree of arm extension.
70% maximum reach must be considered at all times.
Exaggeration occurs when the student fails to relax the sternum, rear knee and elbows.

Long stances look exotic on magazine covers, but the human body gains its power vertically, not horizontally.

Should you be re-assessed?

Ask yourself the following:

1. Have you missed a lot of classes?
2. What was your last assessment like?
3. Did you fail to move up a belt last time you were graded?
4. Do you dread the next grading?
5. Are you training at home?
6. Is the quality of your kung fu not as good as it should be for your current grade?
7. How good are you at the fundamentals?
8. Is your training sloppy?
9. Are you having problems at home or at work?

There is no shame in being honest with yourself. It is easy to remove any pressure you may currently feel.

The highest level of tai chi practice is high stance and small circle. In high stance and small circle you can conserve your energy to a maximum level. This is very crucial in battle. Endurance has always been the crucial key to survival in a long battle. Moreover, due to high stance and smaller posture you can reach to the deepest relaxed stage, the mind is highly concentrated, and the sensitivity and alertness can be extremely sharp.

(Yang Jwing-Ming) 

Training the material you should be training will give you a sense of ease.
If you are failing to maintain your grade, move back a grade.

Instead of feeling pressured and out of your depth, you will feel relaxed and comfortable.
You will be free to work at your own pace. To do things in your own time.

It is important for each student to recognise that you are only as good as the skills you can manifest.

If you wear a black belt but have the skills of a yellow belt, then you are only a yellow belt.
The belt itself is a piece of fabric.
Your skills (or lack of) speak for themselves.

Less effort, more effect...

Every tai chi student must work to reduce the size of their circle.
It is martially imperative for your movements to be small.

You must move without alerting the attacker's nervous system.
Like a shadow. Like a thought.

By balancing frame size, relationship with the opponent and intent, a student can ensure that they employ the optimal framework.
Every movement produces a more significant effect.

The external movement decreases as the internal work increases.
Neigong and intent enable greater effect with markedly less effort.

Instead of sweeping arcs, the student uses twisting, coiling, spiralling action to generate internal pressure in the soft tissues of the body.
These are movements-within-movements.
Smooth, fluid, small, hidden, unnoticed. 

Fixing a problem:

Begin by stopping what you are doing or thinking.
Then, look at your training dispassionately; without ego, pride, vanity or defensiveness.
Recognise the need to address the problem.

If the instructor speaks to you about the diminished quality of your practice, it is important to be honest.
Self-deception is pointless.
Your instructor would not be talking to you if you were not experiencing problems...

Pretending that things are ok (when they are not ok) is foolish.

What matters in a martial art is the effect of your movements.

If you claim to be expressing force yet your partner is physically unmoved by your action, you are expressing nothing.
Your opponent's experience of the kinetic energy you manifest is known as 'jing'.

This is not to be confused with 'li' - hard, skeletal, muscular force.