Why does my hair grow so slow?

Six years ago I cut my hair to chin length. Since then it has only grown six inches. I read that hair grows at an average rate of 1/2 inch a month, At this rate my hair would have grown six inches in a year but it has taken six years to get that length. My hair is naturally curly. I have never colored it and I do not blow dry or straighten it. I take good care of my hair and rarely put it up. I am wondering if this is an indication of a disease. (I also read that nail ridges could be an indication. I have horizontal ridges on my nails.)