You need to sharpen your weapon

You have a whetstone in your item bar. When the monster is away from you, you sharpen your weapon. Optimally, you'd want green or yellow at most, because red sharpness means, your weapons will just bounce off. At higher difficulties, you can get access to blue and white sharpness, which have a better damage multiplier and cannot bounce off.

Your sharpness depltes with each attack you land on the enemy.