The first step I would take is to have the new batteries load tested. A small percentage of new batteries are defective and load testing them is the only way to see if all of the batteries are good, or if there is one bad battery in the pack. If all of the new batteries pass a load test then the battery pack can be ruled out as being the problem and further troubleshooting can be done. Local battery stores such as Batteries Plus will load test electric scooter batteries for free.
Please let me know how it goes.