Hi there I understand your concern and anxiety...you should have physical relationship during ovulation period...normally day 14 is regarded as ovulation day for 28 days cycle...from day 10 to 18 is regarded as fertile period...there are many causes for not conceiving includes
thyroid disorders,uterine anomalies,polycystic ovarian syndrome,certain hormonal imbalance etc..consult nearest gynecologist they will help you further.