Propel Appointments is only able to access events on your primary calendar. Please ensure the event in question is on your primary calendar. 

To check if an event is on your primary calendar, edit the event in Google Calendar then check the 'Calendar' field shown below. It should show the email address of the account you linked to Propel Appointments. 

If it doesn't, create a duplicate of the event on your primary calendar with the same start and end time and ensure that your linked account's email address is shown in the calendar field highlighted above.