Switch the polarity on the motor that is going in reverse.....
If one or more of the batteries fails the load test then it needs to be replaced. For the longest battery pack life I recommend replacing all of the batteries in the pack at the same time and with the same brand of batteries.
If the batteries pass the load test please let me know and then we can do further troubleshooting knowing that the problem is not with the battery pack.
Please let me know if you have any questions.