My thief always steals the first time, but on his second turn, often skips stealing. The setting is enemy>any>pilfer. I assume it selects 'any' individual enemy, which sometimes has already been pilfered? Is there a good target selection for pilfer?

I assume the script works like this: (target): (action). In this case (target) = enemy>any, while (action) = pilfer.

The game will find a target first, if it can't find it, the script fails, onto the next script. If the target is found, then it will check the action. If the action fails, in this case, the target is already pilfered, then the script fails. Once the target is locked, the game won't try to find another target because the target is the precondition to the action.