sex less and less... why?

My husband and I started out real hot and heavy. I'm not typical I want sex everyday, he did when we first got together & multiple times a day. It is coming in on our fourth year of marriage and it has slowly dwindled down to maybe twice a month. It's killing me. Not just because I want it but because when we talk about it he's super into but when it comes down to the moment he says no. He had an awful job earlier this year that seemed to really make him down and depressed. But he switched. We moved. We've had lots of positive things happen the only difference is he works a lot now he is a wrecker driver. Is it just because he's too tired or could there be something else.