Why doesn't he want to have sex?

My husband and I have been married for 8 1/2 years. We just started seriously trying a couple months ago. Anyways lately he comes home from work eats and passes out. I told him this is really important I also told him it makes me feel like he's not attracted to me. He always promises it's not true and we will do the thing tomorrow. why is this happening?? The fertile window is only so big . I'm stressing out so bad!!