Hi, to be able to help you we would need more details, but let me try:
- you either have Huawei, Honor, Xiaomi or Oppo - device by a manufacturer who uses very tough batter optimization. We recommend the website
www.dontkillmyapp.com which is a nice summary of how various manufacturers use optimizing features and how to adjust them, so that your other apps can actually work.
- the other option is that you have Android 8 and more than one VPN app. In such case we would recommend to keep just ours.