Maybe this should be discussed with your doctor, i was told to wait a year before they could test but seeing as you have only been trying with this guy for 4 months im not sure they can do anything at this time. Its also an idea to get hold of Taking Charge Of Your Fertility by Toni Weschler as this will give you a better insight to ovulation and charting, it could be that you are fertile you are just missing the right time to catch, its worth borrowing from the library.
Good luck