We have 2 scheduled reminders set up to send emails to members: 7 days before membership end date, and 0 days after membership end date.
These had been working properly before (in version 4.6.8), but after upgrading to 4.7.15, some members are not getting the emails.
We use Mandrill to send the emails, and looking at the email logs, I saw that only about 1-4 emails were being sent each day, whereas there should have been 50-60.
So I tried changing the "Send Scheduled Reminders" to run hourly instead of daily. That helped, but didn't completely solve the issue. Now it seems that the first time the job is run each day, it still only processes about 1-4 emails. The next time it is run, it processes most of the rest of the emails, but there are still some emails each day that are never sent.
As an example, today there were 62 members whose membership expired, but only 58 were sent the reminder emails. I looked at the 4 members who didn't receive them, but couldn't find any pattern or reason why they wouldn't have been sent the emails. None of the members were set to "Do not email" in their communications preferences. Mandrill records all emails sent out, so it's not an issue of the emails being sent to spam. Also, Civi records the email activity, and none of those 4 members had the email activity recorded.
Over the past week, 408 of 466 members were sent the 7-day reminder email, so that means about 12% of the emails weren't sent.
I've checked the logs for Civi and PHP, and I don't see any messages at the times when the scheduled jobs are run. I also checked the log for the "Send Scheduled Reminders" job, and every time it shows the message: Finished execution of Send Scheduled Reminders with result: Success (1)
I'm at a loss as to why some of these emails aren't being sent.