First of all, you need to check the power bank for its originality [https://www.powerbanktalk.com/genuine-versus-fake-xiaomi-mi-power-bank/|here] is my post about genuine vs fake xiaomi power banks and how to identify one.
Then you can try to put some kind of rubber of padding of paper to make it less movable thus it will be partially fixed. Though there is no permanent fix for this. You can buy new power banks from our [https://www.powerbanktalk.com/best-portable-power-bank-review-2015/|best power bank review list] which are ranked as per performance and portability!