If you are using the original phone charger, did it come in the box new, or was your phone a refurbished unit? You want to make sure it is an OEM charger.

Second thing, some cables do not provide for Fast Charging. If you are not using the OEM cable that came with the phone, then you may have an issue.

It is also remotely possible you have a problem with the OEM charger and/or OEM cable.

You need to check your phone with a known good power supply/adapter and cable capable of fast charging.