It seems there is an underlying fare bucket for which a certain amount of seats can be booked for a flight using FFC's. The number made available can vary by flight. Some flights might not Have any made available.

A booking using FFC's is made as normal against other existing fare buckets but reduces the avalailbility in this specal fare bucket.

Best bet is to book early.

So far I have made bookings on 7 flights using FFC's, even upgraded a couple.