Why does the wall plug charge my phone slowly?

I recently bought a USB wall charger. It has an output of 1.0A and 2.1A, both 5.0V DC. Since I'm only planning to charge my phone (Samsung J7), I probably need the 1.0A; my device then notified me that the device is charging slowly, I switched the cable to 2.1A, but still it's charging slowly. So I was... show more I recently bought a USB wall charger. It has an output of 1.0A and 2.1A, both 5.0V DC. Since I'm only planning to charge my phone (Samsung J7), I probably need the 1.0A; my device then notified me that the device is charging slowly, I switched the cable to 2.1A, but still it's charging slowly. So I was thinking the cable may be the problem.

I tried my sister's wall plug (Huawei) using the same cable, but it charges normally. So what could be the problem?
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