If it is an iPhone: you may have activated optimised charging under Settings -> Battery -> Battery status & charging process. Then, depending on the time of day and user profile, the battery is only charged up to 80%.

In fact, 80% is not the limit but it all depends when the recharge was started. However, it's very obvious that this particular power bank does not have capacity to fully recharge a phone with bigger battery (e.g. iPhone Max etc.).