The Power of skills aren't really meant to naturally level with the user in XP, that is what all the ATK-F, INT-F, etc... are for. They add damage to the skill based on how high the specified stat of the hero is. Naturally, increasing the STR-F would mean that the damage is influenced strength, and so the damage would increase as the hero's strength increases. However, the Power never increases per level. If you are not using XP, I would need to see the skill screen to see the similar function.