I need to correct myself. The tech data from the manufacturer is not complete, so I was guessing there is no Hi-Z input.
There is an instrument input, but the input impedance is not clear, probably 500 KOhms which is a bit too less for low frequency instruments with a passive piezo input. Bass frequencies might get weak with an impedance like that.
Anyway, it rather seems to be problem with SoundJack or the setup of SoundJack.
Hard to help by text, even by remote screen access. But the screen picture might give a first information if something is setup in a wrong way.