Why does my powerbank say in/out when I try and recharge it?

Recently bought a Nankai 20000mAh powerbank. It charges a phone ok but when I plug it in to recharge itself, the screen alternates between In & Out a

Recently bought a Nankai 20000mAh powerbank. It charges a phone ok but when I plug it in to recharge itself, the screen alternates between In & Out and doesn't recharge

or:Recently bought a Nankai 20000mAh powerbank. It charges a phone ok but when I plug it in to recharge itself, the screen alternates between In & Out and doesn't recharge

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