Was he not turned on by me?

I went out with a guy last week, we went out for drinks and he’s a heavy marijuana smoker, I don’t smoke so all I did was have a few drinks. We ended up sleeping together and in my opinion it was the best sex I’ve ever had. He did everything BUT there were times when he’d get soft. I didn’t want to ask him but this has never happened with me before.. was he not turned on by me?