Getting pregnant

Me and my husband have two little girls. We have been trying to get pregnant for about 2 years now and nothing. I track my periods and I also watch for when I am fertile and even test to see when I am ovulating. I have been to the doctor she put me on prenatal vitiums and said everything looks good that there no reason I can't get pregnant? Doctor said I should get pregnant is 6 months it's been almost 6 months. Any tips on what else to try!!!! Thank-you