There would be no adjustment on the output voltage, a VFD is going to fix the output voltage as a percentage of maximum that is proportional to the commanded frequency, it is intended to provide a constant V/Hz ratio to a motor. So if you set up the drive to give you 60Hz max, the voltage should be at max.
I know nothing about that piece of Communist Chinese junk, they are flooding the FleaBay market with low prices and zero support, so to that extent "you gets what you pays for". From what I have seen, you are not alone in your travails about this drive.
But in general, if it has a "Energy Saver" feature, as most do, and you have it in that mode, it may be looking at what it interprets the motor load to be and is automatically lowering the output to be only what is necessary. If it's not in Energy Saver mode, a similar thing may happen if it is a Vector drive and is in Vector mode, as opposed to V/Hz mode. Go through your programming and make sure it is in V/Hz mode and the Energy Saver is disabled.