Even if I turn the HEADPHONES LEVEL knob, the volume of the headphones will not rise above a certain level.

The DDJ-400 operates with power supplied from USB. When the power supply decreases, this unit places a limit on the volume of the headphone output. In this case, please remove other USB devices connected to the USB terminal of your computer. Please do not use your laptop running on the battery. Supply power from an AC power supply.