To explain fully:
BG, Ice/Light/Poison top and bottoms must be 0(0/3) for tops and 0(0/2) for bottoms for this to work.
It will always say 'Success', but may not give extra slot, BUT may reinforce it 1 or 2(0/3). If that happens you have to re-inforce it till it resets to zero before you try again.
Just putting this in plain english.
PS: It may happen easy, or as happened to me on occasion, it may take a lot of stones and reinforcemnt potions to get that extra slot to appear.