I was just poking around the enchantment code in the u3j code (not sure if it is the same as Aion Core), but they do a random number generation and check to see if the success rate is less than the number produced.
The problem is, one of the numbers that can be produced is 100. So, if you set your enchantment rate to 100, there is still a 1% chance it will fail.
You can solve the problem by either setting your slot percentages to 101 instead of 100, or by going into the "EnchantmentService.java" file and changing
Code:
if(Rnd.get(0, 100) < successRate)
to
Code:
if(Rnd.get(0, 100) <= successRate)