Hi, and welcome to the board.
You can not always make a successfull backstab. I have Yoshimo (a bountyhunter) in my party, and he's equipped with a Katana+2 wich he uses for backstabbing, and it works just fine. I think the chances of getting a successfull backstab depends on the level of your character, it doesn't matter what weapon you use however...
I think some weapons are more effective for backstabbing than others:
A quarterstaff might just not be the ultimate backstab weapon!
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...and he walked the earth for centuries, slaying creaturs of the underworld...
-Charlander
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