As always hair are one of the hardest challenge in a portrait and You have to find a way to simplify them, give ovelall shape, tones thus volume, and still suggest, somehow, with masterful traits, that there is any single hair. Therefore artists use to start with main masses and curves and their tones to get some volume to then suggest, with finer traits, group of hairs ... This is the main improvement You may possibly do pretty soon by looking at fine art and drawing masters (for instance google David Malan site). The last one is the best in this sense too, also because hair are emerging from shadow and are less detailed ....
Panta rei (everything flows)!