Why is only one headphone playing and the other not?

Why is only one headphone playing and the other not?
Whenever I turn on the Bluetooth on my Bose SoundLink 2 headphones, only the right speaker works and the sound comes out, but the left speaker doesn’t work and no sound comes out. Whenever I plug in the cable, it’s the opposite. The left speaker works while the right one doesn’t. I think it has to do with the halfway point between the left and right speaker, but I don’t know how to repair it. Please help.

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