Stress can affect your sex life in ways you can't imagine.

Are you doing it the right way too? Is your wife aware of her fertile periods?

There are a few ways to 'test' when the fertile period is. There are ovulation test kits sold, maybe can give it a try.
Normally fertile period is 12-14th day but different people different, so might not be accurate also.

One of the signs of ovulation is that her discharge will be different (more slimy) than usual. She may be able to spot the changes. For me, I sometimes get ovulation cramp on one side of the lower stomach in the middle of my cycle, combined with the fact that the discharge is way more than usual, I know I'm ovulating. Some women also basal body temperature, I have no experience in that so don't know how it works.