Three frequency converters with a set of gearboxes – the frequency won’t increase and the current is very high. What’s going on?

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A group of forty to fifty gears are arranged in a row and mesh with each other; three motors, located on both sides and in the middle, provide the power. The frequency converters used are from ABB’s ACS510 series. The motor power is 18.5 KW. During startup, the current is around 20 amps, while the torque is approximately 30-40% of the maximum value. Once the motor frequency exceeds 20 Hz, both the current and torque start to increase; at around 30 Hz, the current reaches over 60 amps and the torque reaches 80% of the maximum value. After that, the speed decreases, and the motor operates at a constant speed at around 20 Hz, with a current of over 60 amps and a torque of around 70% of the maximum value. The same equipment works fine abroad; the number of gears and the motor power are identical. Could everyone help me analyze what exactly is causing the motor to not rotate? Thank you