I assume that the battery pack is charging up to over 48 Volts but the mileage range is lower than expected. If this is the case then the batteries need to be formatted. To format the batteries simply use the scooter and then recharge the batteries. After the batteries are used and recharged several times they will be formatted. It can take between 20-50 discharge and recharge cycles for the batteries to reach peak performance.
New lead acid batteries need to be formatted before they reach their peak capacity. It is normal for new batteries to only have 3/4 of their normal capacity until they are fully formatted. Below is a chart showing the life cycle of lead acid batteries.
However if the problem is that the battery pack is not charging up to 48 Volts, and it is only charging to 36 Volts, then the problem may be with the battery charger, or with the way that the battery pack harness is attached to the batteries.
I have a Panterra Electric Scooter that I have put a new Battery Pack of (4) Batteries in and I cant seem to get it to take a full charge. It gets to around 3/4 charged and that's it