But why

Britt • Recently married. Pup mom to a silly terrier. Step mom to 3 kids... Trying to figure out my life; and to please God
Everytime my husband and I have sex- we do not use any kind of protection or birth control option. We have done it this way for a year. We have sex frequently- multiple times a day- and he has always cum inside of me.... But I never get pregnant. My period is normal. I have no problems down there... 
He was in a 7 year relationship with his "ex" and they have 3 children together. So obviously he is working just fine... His little one is 3. 
Any idea as to why I can't get pregnant? Is this fairly common?