Could be quite a few things causing this. Again, as in your past posting with this same drive, you are not specific about the actual problem. I assume the motor is slowing down with deeper cuts. Sounds like low motor torque.
1) The VFD could not be setup properly for the motor specs (kw, voltage rating and rpm).
2) Low cost VFD's operate in what's called V/F (volts-frequency) mode, this means the output voltage to the motor reduced as the frequency is reduced to lower motor rpm. When this happens motor developed torque is reduced exponentially... slow down the frequency enough and the motor can stall under load. This concept is a not understood by non electrical people that don't work with VFD's, then wonder why their motor is stalling or runs like crap.
The 750W, 220V motor you had operated at line voltage (220V) and developed 100% torque. I assumed the lathe speed is changed via gears or pulleys. I would use the lowest gear ratio to keep the motor rpm's in the high end to get max torque... then use the speed control pot to "trim" the rpm's to the desired lathe chuck speed.
Industrial VFD's such as Siemens and Toshiba have a "vector mode" of operation to eliminate the torque problems with V/F mode. But you are talking about a $1500 drive instead of a $200 drive. Cheap Chinese drives are even worse for motor torque control and are garbage.