She had an emergency; either at home (she has a new baby) or in the office (chaotic).
She never got the message on her email, never looked at her emails, or lost a paper message on her desk.
She got the message but didn’t have time to call me.
She got the message, saw it was from me, and ignored it (I hope that wasn’t the case).
She had so many messages, that mine never made it to the top of her personal “triage”.
She told someone else to call me back to make an appointment, but they did not do it.
The front desk gave her the wrong phone number.
Hell opened up and sucked in all of the urologists.