I have some males and females of the same kind that won't mate together sometimes you need a 2 to 1 ratio. Just wait for her colors and his head Bob. I find if I put another male in front of the one I want to breed with a female. He gets excited and then mates. If the mouth opens on the female and turns colors and rocks she's not ready.
One or both could be dealing with a undiagnosed health issue (maybe the cham isn't in top condition so never receptive). I wonder if they are responding to our fall season?