After booking an instant lesson, did you send the teacher a message to let them know you booked a class with them?
If not, I highly recommend students message teachers straight after booking an instant lesson because teachers don't receive notifications when one has been booked. It's been raised with support, and so far, italki haven't resolved it.
If you booked a lesson and the teacher told you they were unavailable, you can try and claim the credits back.
Long story short: If you want to have lessons with a teacher, send them a message BEFORE booking a class with them. That way you won't be disappointed. Some teachers also decline lesson requests from students who haven't completed the contact teacher form or sent them a message first, so bear that in mind.