Height and body frame
When you look at other people, and use them as guides as to how fast you hair should be growing, it can be confusing. I used to find myself asking, "How can she have bra strap length hair already and we started our journey around the same time???" I was unaware that some of the people I used as guides were way shorter than me with shorter necks and torsos etc than me :) (I'm 5'7 1/2 with a long neck, and short torso btw lol) 12 inches of hair will not hit the same body points on everyone. Once I realized this, I felt a lot better LOL.
Remember that hair grows on average about a 1/2 inch, more or less, a month :) This varies from person to person. Also, things like genetics, poor diet, lack of exercise, poor regimen, and over all health can affect hair growth. It's soo important to take care of our bodies from the inside out. Just try to enjoy each stage your hair is in.