Hello Morgan

When you set the cron to run twice a day, it checks if there are scheduled notifications to be sent, so when you set "Appointment follow up" to run 15 minutes after the appointment, it will not be sent at that time because there's small chance the "twice daily" occurrence will happen in that specific time.

You should configure the cron to run every 10 or 15 minutes, so it checks:

1. It's 12:59, are there any emails to send? No. OK

2. It's 13:09, are there any emails to send? No. OK

2a. Your customer's appointment ended at 13:15, so follow up should be sent at 13:30

3. It's 13:19, are there any emails to send? No. OK

4. It's 13:29, are there any emails to send? No. OK

5. It's 13:39, are there any emails to send? Yes, Appointment Follow-up. OK, send.

So, the follow-up will be sent 9 minutes later, because at 13:29 there were no emails scheduled to be sent. If you check your email (as a customer), you probably received the follow-up email later on, or it may have even went to SPAM.

If you still can't see it, please provide me a temporary WP-admin login for your site where this happens, so we could log in and take a look ‘from the inside’ as that’s the most efficient way to see and resolve the issue. We do not interfere with any data or anything else except for the plugin (in case that’s a production version of the site), and of course we do not provide login data to third party. 

You can write credentials here just check Private Reply so nobody can see them except us.

Hi again Morgan

It shouldn't slow your website. Crons are run on the server, and they are extremely light. They simply check if there's a notification to be sent or not.

Your cron event seems quite alright as far as I can see, so you can leave it as it is, and it should work.