In addition to what @mytech said You did Reboot to see if you get Fast Charging? Did you inspect the Charging Port to see if there's any obvious damage? You can also go to Samsung Members app / Diagnostics and test the Charger/USB Port 🐧
Sometimes my phone does this and the way I fix it is by unplugging the usb part from the brick and replugging it in and making sure it is all the way in. If that doesn't work, try using a different wall outlet.