The reduced charging rate of your Nissan Leaf could be due to a number of reasons, including:
Low battery voltage: If the battery voltage of your Leaf is low, the charging rate will be reduced to protect the battery.
Charger incompatibility: Your Juice Box 40 may not be compatible with the charging system of your Leaf. It's possible that the charger and the car have different charging protocols, leading to a reduced charging rate.
Charger degradation: Over time, the charging rate of a charger can decrease due to wear and tear.
Cable issues: If the charging cable is damaged or worn, it can also result in reduced charging rates.
Vehicle software update: Nissan has released software updates for the Leaf that can affect charging performance. It's possible that an update has been applied to your Leaf that has resulted in a reduced charging rate.
If you have tried all the above steps and the charging rate remains low, it is advisable to take your vehicle to a Nissan dealership for further diagnosis and repair.