I can view some slots of my Educator when I select 30-min duration. But when I select 45-min duration, the slots disappear. Why is this so?

This means that our Educators can only teach a certain number of classes in a day. When they reach their maximum capacity, they will not be able to accept any more bookings for that day. If this happens, you can try reaching out to your Educator to see if they are able to adjust their schedule to fit you in, if they are available.