Which version are you playing TC? Because on the PSX version or any version like that, running is random.

However, on the nes version, if any character's luck is 15 or higher and they are in the 1st or 2nd slot. They will run away 100% of the time. Except against bosses and some other enemies.

So if you are playin the nes version you should move your thief to the 2nd slot.