The Ditto's output maintains the signal level as it gets from its input. The level knob acts only on the output both for loops or track level. Please try to boost your signal to the input of the Ditto using a booster pedal or a preamp. Another option is to normalize the signal once you've exported the audio file on your computer with the help of an audio editor like Wavelab, Audacity, Soundforge, etc.
Keywords: Ditto, X2, Looper, X4