Sorry I’m only just replying to this!
The motor driver has a current limit of 1.5A. If this current is exceeded, the power is cut for a short time. Depending on the size of the motor, the startup current might exceed the limit, so the motor doesn’t start.
Having said that, the fact the USB connection makes a difference suggests it’s an under-voltage problem (i.e. the voltage drops too low when the motors start).
Do you have a link to the motors you are using?
PS There was an older version of the software that didn’t output full power to the motors (bug). So might explain why it was previously working at 100%