You should seek some medical advice from a doctor who understands Catholic Moral teaching and find out if there is any medical issue preventing either one of you from conceiving. [Other sources.] You can also try Natural Family Planning techniques to determine when she is ovulating and the best times to try and conceive. Do a little research on the internet.
You should also pray — with your wife — together, for your mutual fertility — and God's will for both of you. God is the author of life and this is a 3-way deal. It is not up to either one of you, but Him.
Remind your wife that she married a husband, not simply a sperm donor. There are no guarantees about fertility in a marriage, and certainly circumstances vary from time to time making conception harder to come by. Many couples have to wait years before it happens.
My wife and I could not conceive because my wife had a medical issue, so we adopted. Sometimes God works in different ways. I know a couple who couldn't conceive, adopted, and then got pregnant a year later! You just can't tell what will happen.
Just pray and do some homework.
You may need some marriage counseling if she doesn't seem to get it.