First determine if there is still electricity in the charging bay. The charging bay is red as shown. It shows that there is not much electricity left in the charging bin, so you can charge the charging bin first. When the charging bay is fully charged, put the headphones in. If it still doesn't charge, check the contact of the charging bay. Mouth is not a problem, such as wear or dust, if there is no problem, contact after-sales.
The Xiaomi air2s headset I bought also appeared in this situation, one ear could not be charged, and then it was fine after replacement.