Where are you, advisee?

Where are you, my advisee?  Each semester, the 75 or so students I advise are supposed to stop by to discuss their program and get the PIN they need (personal ID number) to register for the coming semester. I send them all an email each semester inviting them to contact me. But only about half ever do.  How they get the information and PIN they need, I don't know!  Probably from the department chair or other professors who happen to be available when I'm not.  I worry a little bit about these phantom advisees.  But only a little.  I assume they know what they're doing.