my erections arent as hard anymore

I'm 17 and I have noticed that when I get an erection it doesn't get as hard as it used to, sometimes it isn't firm enough to have sex. I don't think I over masturbate, I have probably averaged once a day for the past year atleast. Some weeks I used to not masturbate every day, but other weeks I'd masturbate a couple times a day. I can still ejaculate even if it is half erect. Any help ?