Debuff chance decrease drastically if lower lvl skill is used on higher lvl target.
To make it not decrease, you have to enchant skill.
It doesn't depend upon your lvl, only on skill lvl being used, which is 76 for +1 2nd occupation skill, and 81 for +1 3rd proff. After that, each +3 to enchant gives +1 lvl, so steps are +1/+4/+7/etc.
