Appointment Reminder only syncs appointments from Google Calendar where we find a mobile number or email address in the appointment somewhere. (as these are the only appointments we can send reminders to).
Note: We can sync all appointments if you want. Go to Settings > Edit Calendars > Edit and turn on "Sync All Appointments" however there isn't usually any point in doing this unless you are using "Online Booking" which requires us to sync all appointments so we know which time slots are available.
It can be anywhere in the appointment details, subject, location or description fields.
Also by default we don't sync All Day appointments from Google Calendar. To enable this go to Settings > Edit Calendars > Edit and enable "Sync All Day Events". If you are sending reminders for all-day events you won't want reminders being sent 24 hours before the appointment. Best to change the send time in Settings > Business to Custom and set days before to 1 and the time of day to something like 08:00 for 8am. You can also change the send time for each calendar by changing the send time in each calendars settings. Settings > Edit Calendar > Edit.
Please note: It doesn't matter how you enter the mobile number in Google Calendar, just write it how you would normally write it, our system will be smart enough to figure out if it's a phone number or not.