In my case, the working hours start at 10AM every day, for almost all providers and all of them work with 60/120 minutes-long appointments.
The providers of a specific service work three days a week (Monday, Wednesday and Friday) from 18:00 to 19:00, and Tuesday and Thursday from 17:30 to 18:30. My schedule shows Monday, Wednesday and Friday without problems but on Tuesday and Thursday, the free time slots don't show.
I set the Time base option to 30 minutes.
You can see the problem here:
The service that have the problem is "Surf-Fit (niños)".