So we had to buy a new cable to charge our phones, the one that came with them has worn down. Just charged my phone over night, already optimized and did a reset earlier in the day; it was at 33%. 5 hours later, with the lightning bolt on the battery symbol, its at 30%. Its USB-c and a third party charger offered by a major retailer. I think the cable should work just fine and not be ignored by the device. Same wall plug, just a new cord. Wasn't the cheap $5 kind, but a reasonable $20. Why is this such an issue?
Make sure it is an Adaptive Fast Charging or Quick Charge power adapter. Make sure the cable is rated to output at 3 Amps or more. Your phone should say Fast Charging in the notifications while charging. If it doesn't you need to enable it in settings. If it is enabled and doesnt say fast charging then it's your power adapter or cable or both aren't compatible. I got one off Amazon for $9.99 and it Fast Charges my S8 Plus. I know everything about Samsung Phones and charging technology. There is a chance it could be your battery is going bad but most likely not.