Discharging issues can occur for various reasons, the most notable being too bright a screen.
It could also be excessive background activity which can be observed by going into Settings > Battery and Device care > Battery. Use this to determine what apps are consuming the most power and whether it's viable to limit their background usage. (Sleep/Deep sleep)
Next I'd suggest running the optimisation service found in Settings > Device Care. It's worth toggling the auto optimisation on whilst you're here.
Finally, the worst case scenario is that the battery has reached its' life cycle.