Why do reminders not go out to some of the contacts who should receive them
One reason could be that an existing Reminder had its criteria changed!
EG ...
X was sent a scheduled reminder for reminder no 14 on 23 Jan 2015 and the entry relates to his membership end date being 31 Dec 2015.

At the time the reminder was sent scheduled reminder no 14 was configured to send a mail 23 days after the expiry date (then 31 Dec 2014). On 25 May the reminder was changed to send out 7 days before expiry date.

The reference_date column was not added until 28 August and at that point the current end_date was stored for the reference date. There was some calculation that went on regarding this but to the extent that it was correct in general terms, it was messed up by this reminder's history.