It could be one of many causes. Recently, on the Accordion Repair Group’s Facebook page, a similar problem was solved with a Y-adaptor on the amplifier end of the cable. In that case, the accordion had a stereo jack, the cable was a stereo (TRS) cable that needed the adaptor to convert the TRS into two TS plugs. That was an easy fix. But your problem could be anything from a broken internal wire to a bad volume control to a bad jack and, finally, a bad mike. An accordion repairman who’s familiar with this model should be able to fix the problem. If there isn’t one in your area, an electronics technician might be able to help.
Good luck!