We would start by testing the charging system. For this test plug the charger into the into the wall, and then plug the charger into the scooter while watching the lights on the charger. The charger's light should turn from green to red as soon as the charger is plugged into the scooter.
If the charger's light does turn red during this test, then the next step to take is to give the battery pack a full 48-hour charge to equalize the batteries. During this 48-hour charge, leave the charger plugged into the wall and scooter even after its light has turned green. Then after the 48-hour charge, take the scooter out for a ride and see how it performs.
Please let us know how it goes.
Just replaced batteries and scooter almost immediately shows low battery when I ride and has minimal power