Hmmmm....
Try wiggling the plug in and out of the socket on the Powerbook.
My adapter is a little damaged and I must make sure it is just the right place to get it to work.
My girlfriend's iBook adapter went bad and Apple replaced it.
But it could be the Powerbook. Try resetting the PMU with the Powerbook unplugged. Then plug in with the machine still off and power up.
If none of that works, just take it to Apple. You have a one year warranty on that adapter. They should give you no hassle for that.