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.  Go to Calendar View, and then check your filters on the top right. You may have the filter set for Paid/Unpaid, team view on, or maybe even the cleaner is unchecked from the filter.

2. Be sure to check all filters just in case 

2.  In Settings -> Scheduling -> Calendar appearance, look at the beginning start and end time set for your calendar.  So for example, 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.