Wife is finding it hard to concieve?

Hello everyone .Im a father of four lovely amazing children and married to the love of my life for ten years! Our four children are 9, 7, 5 and 4.I myself am 43 and my wife is 32 over the last year we have been trying to make a baby and unfortunately luck is not at our side at this moment .I hate the feeling i get each month when my wife starts crying says she isn't pregnant because her mood changes instantly, with the last 12 months this has been happening and it upsets her makes her angry and not in the best of form i try to be supportive to her it hurts me to see her upset like this.With all the other births she became pregnant Pretty fast!but this time its opposite.We are going to take custody of her late sisters boy she died from cancer and we are hoping the court will grant custody to us as her and her sister were very close ,we love children very much i have explained to her to go to a doctor and see if something is wrong but she wont listen to me ,although we may be receiving her late sisters child i still am curious about why we aren't conceiving.she is 32 is it her age or maybe my age im 43 that's causing this? or does anyone know of any other possible reasons that we can't have children anymore? she won't see a doctor because she thinks it is an embarrising topic to ask them which in my opinion is very silly but i just have said it so many times to her now and i gave up .Im am very grateful for any advice or answers thank you very much for your time.