Why a particular time is showing as "Not Available"?

Here are the steps to troubleshoot availability

1. Check Business Hours: See if that time comes under your Business Hours from SETTINGS-->BUSINESS HOURS. 
2. Check Existing Appointment: Check if any other appointment exist at that time. 
3. Check Blocked Times: Check if that time is not blocked for that particular staff. SETTINGS --> BUSINESS HOURS --> FUTURE UNAVAILABILITY 
4. Check Allowed Dates: Check if you have not set a date has the only allowed date for that particular staff. When you set a date as allowed, all the other block dates settings will over ride.
5. Check Time Slot Settings: Check if your time slot settings is correct from SETTINGS--> BOOKING RULES. For example, if your time slot is set to 15 minutes and your service lasts for 20 minutes then your service will never show 10:20 AM if there is a booking at 10:00 AM. Technically, your service will last for 20 minutes and your settings say that you want to show time interval on 15 minutes scale so 10:20 will never be shown in this particular case.
6. Check Appointment Lead Times : Go to SETTINGS--> BOOKING RULES-->Appointment lead and cancellation times and see if your lead time settings are correct.
7. Check Precision scheduling setting: Check if you have enabled precision scheduling and the time showing as "Not Available" is within the range.