You have this problem, I believe many people have encountered. If it is plugged into the socket to charge, there will generally be no phenomenon you say, unless the non-charger is unqualified, the quality is not very good charger will indeed suck power, because the charger itself needs to consume power, in the absence of plug AC support will in turn consume the phone's electricity.
Also, if the charging adapter is no problem, replace it with a data cable to try, there may be a short circuit in the data cable connection. Battery damage may also cause this phenomenon, if it is not possible, can only replace the phone battery. Therefore, when charging the mobile phone, be sure not to play while charging, be sure to charge with the original charger
This situation should be that your charging bank switch is not turned on, some charging bank plug in the receiver, the switch does not automatically activate, in this case there is no boost function, only the voltage of the battery itself, when the battery voltage of the charging bank is lower than the voltage of the mobile phone, the battery of the mobile phone in turn to charge the charging bank, so plug in the charging bank to see if the charging is OK, if there is no reaction, You have to hit the power bank switch