As Samsung has enhanced the charging algorithm to latest software version, using unapproved charger or cable to charge Galaxy S8 / Galaxy S8+ devices will take longer depending on the charging conditions.
It is strongly recommended to use Samsung-approved charger and check if it is connected properly to the device. Connecting the charger improperly may cause serious damage to the device as well as slowing down the charging speed.
If any foreign materials were inserted into device’s multipurpose jack, fast charging will not work normally to prevent unexpected device damage during charging.
If the issue continues to persist while using Samsung-approved charger and cable to charge the device, please visit a Samsung Service Centre with Galaxy S8 / Galaxy S8+ device, charger and cable together for further investigation.