Hi...it might be due to uterine abnormality,hormonal imbalance, fibroid in uterus,sometimes infection like T.B would have scarred fallopian tube which might be a cause,certain medications, obesity etc. In men- decreased sperm count, chronic alcoholism,smoking,obesity,premature ejaculation,medications, testicular infection or trauma etc....so get this things checked. You should know that there are 20 to 25% cases where the cause is unknown for infertility....As I already mentioned if you are not concieving for more than one year, you can try invitro fertilisation(IVF) technique which has a good sucess rate....kindly consult a obstetrician regarding this they will guide you further