Your NCO should be counseling you as to why they feel you are not ready for the promotion board. And that counseling should identify areas that justify why they believe you are not ready. To acheive that next skill level, you must have mastered the one you are in now, so you can lead and train those that are subordinate to you.
My advice is to follow your NCO support channel, and don't by-pass those NCO's in it. That could compound your situation.
Whom ever counsels you, it is their obligation to you to bring it to your attention the areas they feel you may need some work in to attain that next level of leadership. You are a reflection of their leadership, and it appears your ambitions are of good nature, but ensure that you are staying with in your bounds to reach your goals.
If, and only if, your counselor and mentor refuses to annotate on a 4856 why they are not recommending you for the promotion board, then I would utilize the open door policy. But try and take it up with them first, you are allowed to ask questions at your counseling session, so ask!
Keep going to the SOM boards. They are good learning platforms that will develop you and prepare you for your promotion board. And when your time comes, you will knock your promotion board out of the park.
Even after the promotion board, continue going the SOM boards. They show ambition and a will to excel in what you do.