Health steal enchantment

What am I doing wrong here? The idea is that the weapon with the enchantment will have a chance to reward back half of the damage dealt when you attack, unless the enemy doesn't have that much health in which case it will just give the player the health the mob had before being killed. The problem is that it doesn't always work, sometimes the game will show you as having full health when you should've only been given a little bit (I believe it happens most often when the mob is killed) but when you take damage you lose all of it as if you never gained any health. The latter part happens when it successfully rewards you the amount it's meant to as well, but for some reason it works sometimes and doesn't other times. Does anybody know what could be causing this? I'm at a loss with the procedure layout. Thanks