It's not uncommon at all! It's far more common in people with a vulva than people with a penis, but can happen to either. It's often related to how comfortable and relaxed you are in the moment, which is partly why it is easier for most people to orgasm alone than with a partner. With masturbation, sometimes switching it up is all it takes - add a new toy or technique to the session, or even try watching something that arouses you that you haven't masturbated to before. Make sure you're comfortable and aroused, and don't get frustrated or annoyed, be patient with your body.
Here's a helpful article about masturbation and orgasm:https://www.scarleteen.com/article/sexuality/sexual_response_orgasm_a_users_guide