Hair grows

Hair  grows at different rates on every part of  our body including our scalp. Hair goes through  three phases of growth and each area of our body has different phase times. We can't do anything to even hair growth times but regular trims will keep your style looking consistent. 
Within the three phases of hair growth are anagen, catagen, and telogen.  Anagen is the most active stage. If someone has a difficult time growing their hair beyond a certain length, its because they have a short active phase of growth.  On the other hand, people with long hair  have a long active phase of growth. The caragen phase is the transitional phase and about 3%  of all hairs are in this phase at any time.  This phase can last between two to three weeks. Telogen phase is the resting phase and usually accounts for 6 to 8% of all hairs. This phase can last up to 100 days for hairs on the scalp and longer for hairs on the rest of the body.
Hair on the scalp grows about 6 inches a year.  Hair growth  and shedding is random and not seasonal and cyclical. 
There are hair systems that will help stimulate, revitalize, and encourage healthy cleansing as well as conditioning.