Grade Drop Bug

I have now one Problem,with drop.
Today change my grade % in general mobs and Boss!!
Before drop ok! but now I have problem in Grade 31,I just make update and sql but not drop grade 31! change every items from grade 31 to grade 33 making update and sql again,and continue not drop!
Anyone have idea what happen??
Thanks bro!

check the database for mobiems and check grade 31 because as long as you done it right with right Drop % it will work i no certin items wont drop from grades higher then 999 but then some do