TTC in late 30s ?secondary infertility

Good morning Ladies

I have been TTC for over a year now. I have two older girls but I am desperate for another one! I have monthly periods and have never missed one but still don't understand why I have failed to conceive! What could be the cause? Is anyone of you ladies experiencing this? Please share your experience. PS I have recently turned 38 years. My periods are heavy enough and have never missed any.