They mean, bluntly, that they see you are not displaying enough knowledge of the process, ability, or talent. Such that you might be perceived as more of an effort to train than it is worth.
This might not even be justified or correct. But something to understand is that they are very practical-minded. They have a work-process. And that is about 90 % of the job. And art-ability is only about 10% of it. That does not mean you have to be 1/10th of a good artist. That means you may have to be 300% better than anyone. But even then, it will only take up 10% of what you have to know to do the job.
It is that other 90% that makes you a senior over a junior. And getting in as a Junior means your talent has to be displayed such, that you are above average. Or the best they can expect is a below average artist that they have to train to become eventually a senior, and even remain, below average.
So what should your focus be? Making kick-ass art. Or at least display a high level of competence.
There is nothing wrong with your portfolio in terms of quality. Everything is professionally presented. But if there is anything missing is the wow factor. Something that stands out that shows your personal talent or style or an ability to tackle challenging work.
So the best way to accomplish that, is to find some artist that you really admire, and try to copy it. Take something you think you can do, and figure out how that artist did it. And then do it. And keep challenging yourself above your current ability this way, copying other artists work.
And in the process you will start learning the production pipeline. Like UV maps and textures and retopo and all of the rest of it.
And as a bonus you might even discover you have a style.
Then put together a killer portfolio that shows you can tackle challenging work, your style if you figured that out, and that you understand the process, by showing breakdowns of how you did it.
The rest will take care of itself.
