It sounds like you may have multiple PBS accounts and are signed in with an email address that is different from the one you initially used to activate Passport. The email address you have on file with your station, (i.e.: the email address you used when you made a donation) is often not the same as the email you used when you activated Passport. Try a different email address when logging in to see if it works.
If your Passport is activated properly, you will see a small blue and white "compass rose" icon ( ) by your name in the upper left portion of the screen.