Hair is 50% genetics and 50% feed and enviorment.
SOme animals never have any hair. Others have it year round.
Obivously the higher the corn ratio in your feed or the hotter it is the less hair you will have.
work, work, work. Also as OH B says, if genetics not there hair growth limited. some swear by products like Shag but I've found rinsing, brushing & fans or cool room best.