A very simple reason could be the poor diet that lacks minerals and Vitamins. There are innumerable other reasons but will keep it simple to start with.
Next Steps
Add Vitamins such as A, C, E and K that are good for skin. Add Copper and zinc that are essential for good skin. You can read online about a diet which is rich in these. Consult a doctor if the situation does not improve. Follow health tips below
Health Tips
Stop applying any Over the counter Hair Growth medicine and Shampoos meant for Dandruff /Hair Growth treatment . You may be sensitive to some hair care products and damaging your hair/introducing more danduff. Exercise for 30 minutes a day ( e.g. cycling ). Keep your hair clean. Brushing/combing is essential. Chronic Stress should be avoided. Dont sleep at odd hours. No Pony tails/Tight Hair Braids.