first we need to know if it is the same in both directions, if so, you most likely have worn out the brushes in a drive motor. if it only happens in 1 direction, then it can most likely be the contactors.
what is happening is that like a car with a differential, the inside tire in a turn must turn less RPM than the outside tire, this is done by cutting off power to the inside wheel, if the outside motor current is not good, there will be no drive at that point.
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