I had a misscarriage 6 months ago and been trying again for 3 months almost 4 nothing is happening why is that?

It can be just luck. Did you know that one-in-three to one-in-five pregnancies miscarry? That may not offer a great deal of comfort, but please don't give up. Learn all you can about timing intercourse with your ovulation ("take charge of your fertility" by weschler) to maximize your chances. See your doctor if nothing is happening over the next few months, or if you have had several miscarriages. Good luck.
Ovaries. The answer may be the ovaries are not working the way they should. Most miscarriages occur due to genetically abnormal eggs. That can occur with increase frequency in some women, even under the age of 20. So either you can wait a couple more months and see what happens or you can always see a fertility specialist.