In my game, Nalia's been able to find almost every trap...it just takes her forever to do so. Master Thievery potions help...so does the "Detect traps" spell or something like that. If you use Jaheira in your party, I know she has some kind of "Detect Traps" spell, so you may want to use that as backup as well.
Other than Edwin (who's evil), there is a significant lack of good mages in the game. Since I play a "Tank" as my main character, I usually take Nalia to begin with. She's available very soon on into the game (a great replacement after losing Imoen). She's a great mage and her usefulness in that area lasts for quite a while.
