It takes some time for your body to grow up and start producing sperm. Even if you have already hit that phase of puberty when its producing sperm, you may not cum if you masturbate too often, or if you're not very sexually excitted while masturbating. For example if you finish very fast without being very excitted you may not cum. Most likelly you haven't reached that phase yet when you can cum. I started cumming only a year or 10 months ago...
it's very normal. Just give your body some time to produce new spermcells. Try not to masturbate for 2 weeks or so. Try again,I will ansure you that you will cum
Most likely it is because you just have started to produce semen. I am sure you will start producing more now that you are cumming more often. Nothing to be worried about.