Issue about guitar volume thrue BB

Hello. I recently bought a BB and and I have a problem.
When I plug a guitar in mono Left input of the BB, sound level of the guitar is very low, sometimes near 00, sometimes a lot of distorsion
I check with différents guitars, cables. All is all right when I direct plug in the amp, problem remains when I plug in the BB.
If I plug in the right input and output, it’s better, perhaps running fine and normally.
Is there any option somewhere to resolve this problem ?
So, I can use it but not in the standard way (mono left).
Did someone face to the same problem ?
Thanks you all if you can help.
Have a nice day and fly safe

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