Husband has stopped going down ??

So a few years ago my husband asked why I wount go down on him an I told him that I didn't know what I was doing an wasn't sure he liked it (he did and does) so I did it more often. He would always go down on me and it was amazing. Sex is amazing. About 2 years ago he just stopped and when I ask why he blows it off. It hasn't been that long or I'm tired or something. I'm clean and not smelly so I know that's not it. He has no problem asking me or trying to get me to go down on him. But now it's just turned me off all together. (15 years together)