Sacred Flame is based on Dexterity, not Wisdom (it says so in the tooltip). If you don't have a lot of Dexterity then Sacred Flame is only useful against sleeping/unconscious/prone targets.
Look at it closely when you actually attack with sacred flame, if you check the combat log and scroll over the damage, or miss in most cases unfortunately. You'll see the Spell DC for sacred flame with the cleric, lets say its 14. If the enemy has decent dexterity, then when they roll and have a good modifier, it'll really help prevent the attack from happening (saving throw much like you hitting with a weapon with ac for example, if the number is the same it's a hit, and for saving throws if the number is the same or higher it'll prevent the attack from happening) Before you use sacred flame, take a look at the enemy and examine them and look at their dexterity to get an idea of their modifier. Using a warlock's hex: dexterity will put them at a disadvantage for dexterity saving throw, which does help but won't make it 100% chance to hit like causing them to be prone.
