I have a fighter/thief and when i get behind an opponent to do the sneak attack it doesn't always work. I have gotten itto work a couple times, but i'm not sure on wut is wrong? Thx!
That depends on the dice roll ie initiative, will, reflex etc, and also on the level of your thief. The higher the level, the more feats he acquires. But more important is the random generation by the computer. Turn on the dice roll and you shall see when it is successful.
Sneak attack depends on the thief's ability to move silently and this is related to dexterity and intelligence/wisdom. If your thief has high scores in these attributes, then the sneak attack will succeed. the damage done goes up with the level of the thief, usually every 2-3 levels.