What is the reason for a lack of charisma? It is said of a very charismatic person that when they walk into a room, you only notice them. They magnetizes us.
They stand straight, shoulders back, walk with presence.

Also, we say super charismatic people (take Donald Trump for example). It is said that when he comes to you  and looks you straight in the eye, it feels like the only person in the room because his presence is so strong.
We notice this person as soon as he enters a room. Conversely, a lack of charisma would have the opposite effect:

You don't necessarily notice the person when they walk into the same room as us.
It doesn't necessarily give off something, or something “positive and magnetic. You don't feel drawn to this person.
They do not  take up much space. They're  not the person you will notice if they're  not there.
They're  not going to look you in the eye making you unique wishing all the best / asking for nothing in return
In short, They don't have all the little habits that a charismatic person can have.

Think of  a lack of charisma as something that's mostly due to a lack of awareness of what a charismatic person is.

And things that make a person charismatic or not.

The person will not be aware that they have to stand up straight, that posture and body language represent 55% of communication.

They're not going to know that a fake smile will be seen and felt by most people.

They're  unaware of the importance of remembering first names
They're  not going to realize that a person is going to feel important if you care about them, really listen to them, and are 100% present with them.
Etc., etc …

We say that to acquire a skill, you have to go through 4 stages:

Unconscious incompetence (we don't know we don't know)

Conscious incompetence (we know we don't know)

Conscious competence (we know we know) but we still think a little - with repetition

Unconscious competence (you don't know what you know) it has become a habit

-> If we start from the principle that charisma is a skill and can be worked on (which we strongly believe).

So we can say that the lack of charisma is due to the unconscious incompetence of the person.