There are a few reasons that a customer's appointment is not showing up in the calendar when in fact, it does exist in the system.  Check for the following:

1.  In the Calendar View, upper right, check your filters. You may have the filter set for 'paid' or 'unpaid', team view on, or maybe even the employee is unchecked from the filter.  

2.  In Settings>Calendar Options , look at the beginning start and end time.  So if you have your start time set to 9:00 am and the appointment you can't find starts at 8:30 am, it will not show up on the calendar, even if it is actually scheduled for your customer.    

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