The unlocking failure after plugging in May be caused by the fact that the charger you use is not the original charger. When the iPad is charging, if the original charger is not used, the screen may be black or the fingerprint key may fail.
At this time, change the original charger charging cable, if it can not be solved, empty the background restart, update the system to the latest, if not, you can only take off your iPad case, one hand to touch the metal behind, one hand to unlock, this situation is the back shell static induction interferes with the fingerprint unlock module, use your body to release static electricity.
Because the capacity is relatively large, there may be no way to unlock it when charging, as long as it does not affect normal use, there is no problem, but this situation may be caused by the problem of the charging cable, and you can still go to the repair shop to check the screen if there are conditions.