My comment would mirror Chandra1313.
It has been my experience that, unless a horse rubs or has an issue causing hair loss, their mane is what it is. We have 4 horses. 3 have manes that grow to about the lower part of the neck and 1 gelding that has a short, sparse mane, and that is just the way it is. Also, some breeds seem to ge gifted in the mane and tail department while others can be a bit short changed. Often we just can't fight Mother Nature.
But good luck...maybe someone will have a miracle solution here.
If you ever find yourself in a fair fight, it's because your tactics suck. ~ Marine 1SGT J. Reifinger