My Guy Who Has A Very Big Sexual Appetite Wonders

My guy who has a very big sexual appetite wonders why when he kisses my breast or plays with my nipples they only stay hard for literally 10 seconds? I remember when I was younger (I'm 34 now) when any little movement made them stay hard and now even though I'm aroused, cold, etc they will either not at all or only do so for a matter of seconds. My guy thinks I'm not turned on by him, but I totally am. It makes me feel like something is wrong and that I have nerve damage somehow. Please help!