Most people embarking on a screenwriting career don’t do anything about it, so their chances of success are nil.
Corey Mandell, an L.A based script instructor has simple advice. Talent is innate, but many of the craft based skill sets in writing can be learned. Most writers are unaware of which paradigm they write in and how they can improve.
Write consistently. Treat writing as a profession rather than a hobby. Repetition is the key to improvement. Don’t spend years on one script. Set a reasonable deadline of 3 to 6 months per script so you become prolific over the years.
One key aspect that many writers don’t grasp is that the reader is their first audience. They need to write COMPELLING CONFLICT to ensure readers remain engaged and keep reading. Is there enough conflict in the story? Does the reader experience the same movie that you wrote?
CONCEPTUAL vs INTUITIVE WRITERS
Conceptual writers write from the outside in. They have a premise/concept in mind and create characters to service the plot. They write from a place of structure, logic and efficiency. The story must be figured out before they write it. Conceptual writers operate from their left brain. They are logical, efficient and worry about the past, present and future. Conceptual writers are product/results oriented.
Intuitive writers write from the inside out. They begin with emotion, character and theme and develop a plot from there. They write from a place of truth and experience. They must write to find the story. Intuitive writers write from the right brain. They are holistic, ephemeral and scattered. It is difficult to hire these people on rigid open writing assignments. Intuits are process/ experiential oriented.
CREATIVE INTEGRATION
Most writers are unaware of which process they are using. If they are predominantly one, they can train themselves in the other. In order to become a better writer, write to your strengths and avoid your weaknesses.
Human cognition doesn’t allow both techniques to be used simultaneously. That is why you can’t write your thoughts from a stream of consciousness and outline plot and structure at the same time.
Once you figure out what type of writer you are, try to write in the opposite process. It will enhance the overall quality of your predominant writing style by exercising your creative muscles.
Although screenwriting paradigms by the established gurus have their place, slavishly adhering to them won’t get you noticed. Imagine if there was only one type of cheesecake? Delicious but boring. Uniqueness and individuality within the parameters of the production pipeline will.
Gideon's Screenwriting Tips: Now You're a Screenwriter 
These are some wonderful points to make. Another reason that I believe so many writers fail stems from the idea of approaching it as a profession as a hobby, but I think it often has less to do with the writing part and more to do with the self promotion part. It takes dauntless time and energy to make the calls, pitch the work and remain steadfast in spite of rejection.