Not enough sex?

When me and my man first got together, we had sex.. A LOT.. We've been together for about a year and a half now and we have sex maybe once or twice a week. He used to tell me he used to have a really high sex drive. I don't know if it's because of his new job keeping him tired and stressed all the time or if I've done something wrong. To be fair, he does work a third shift job and has to walk about 10-15 miles per night. I guess my question is, has this happened to anyone else? What did you do about it?