Happy to help:

Tell the agent that he has to make a so called "Blind Booking"!

Usually FB cannot see the (award) availability when checking the flight, they have to make a booking (X for Eco, O for Business Class), when the sent the booking within a few seconds up to two minutes the KE system confirms or deny the booking. The FB booking machine has improved in the last months, so you might be able to book the award as well online. Unfortunately when you check the award availability on the KE website it shows you availability; however it does not mean that you can book an award with FB miles. FB has a limited number of seats in their contingent, if those seats are taken, they will not be able to book any awards seats for that specific KE flight, even the award booking class this flight is still available. I had a very long conversation with a KE employee via the call center who explained that to me last week...

I hope this helps..

Good luck