The culprit for slow hair regrowth this long after treatment might be lower estrogen levels if chemo put you into menopause or if you are taking a hormone therapy.  Many menopausal women experience hair thinning.  Continuing to wear your wig might also be compounding the problem.  Your scalp needs to breathe, and the pressure of the wig might be breaking off the fine hair that usually comes in first after treatment.

My hair dresser recommended a line of hair products called Nioxin, but I couldn't see that they helped me much.  I've heard other women say that they had good results with Nioxin.  I believe that there are other brands of similar products.  Your hairdresser would be the best person to help you find products to make your hair thicker or to find a hair style that gives the illusion of fullness.