You'll need to provide a lot more information for anyone to give you much help. You use the term "airport network," so I'm assuming you're using some kind of AirPort Base Station-which one? Which models of iBook and PowerBook do you have?
What are the differences between the two computers? Are you able to connect the PowerBook via an ethernet cable? Does the PB's AirPort icon show the same network name all the time, or is there some other name.
And in spite of changing from titanium to aluminum, the PB's case is still something of a problem in terms of wireless connections, so it could be you're just trying to use it farther away from the base station than it wants to connect.