Check the Error Log doctype / table for clues.
In Error Log not any error related to notification and days before and after the field. I will give 0 days so the notification comes will be the next day not come the same day.
Facing the same issue. There are no entries in the error log, the scheduler is running as it should be and when setting up the notifications it even shows the notifications to be send out. The conditions seem to be working. Unfortuantely, no e-mail is being sent out. All other notification settings (like after_save etc) work just fine.