The solution to the problem is simple, take a wet cotton then rub for 1 minutes on the charging pins of the band, then charge it… voila!!!
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Edit (2023): If still not working, just take a scissor or any metal and gently scratch the charging pins, this will remove the thin film formed on the charging pin due to sweat and moisture. Wipe it off with wet cotton or alcohol (if you have). Then try to charge it again. 🥳